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6,401
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_3
| 1
|
convex polyhedron $P$ has $26$ vertices, $60$ edges, and $36$ faces, $24$ of which are triangular and $12$ of which are quadrilaterals. A space diagonal is a line segment connecting two non-adjacent vertices that do not belong to the same face. How many space diagonals does $P$ have?
|
Every pair of vertices of the polyhedron determines either an edge , a face diagonal or a space diagonal. We have ${26 \choose 2} = \frac{26\cdot25}2 = 325$ total line segments determined by the vertices. Of these, $60$ are edges. Each triangular face has $0$ face diagonals and each quadrilateral face has $2$ , so there are $2 \cdot 12 = 24$ face diagonals. This leaves $325 - 60 - 24 = \boxed{241}$ segments to be the space diagonals.
| 241
|
6,402
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_4
| 1
|
Square $ABCD$ has sides of length 2. Set $S$ is the set of all line segments that have length 2 and whose endpoints are on adjacent sides of the square. The midpoints of the line segments in set $S$ enclose a region whose area to the nearest hundredth is $k$ . Find $100k$
|
Without loss of generality, let $(0,0)$ $(2,0)$ $(0,2)$ , and $(2,2)$ be the vertices of the square. Suppose the endpoints of the segment lie on the two sides of the square determined by the vertex $(0,0)$ . Let the two endpoints of the segment have coordinates $(x,0)$ and $(0,y)$ . Because the segment has length 2, $x^2+y^2=4$ . Using the midpoint formula, we find that the midpoint of the segment has coordinates $\left(\frac{x}{2},\frac{y}{2}\right)$ . Let $d$ be the distance from $(0,0)$ to $\left(\frac{x}{2},\frac{y}{2}\right)$ . Using the distance formula we see that $d=\sqrt{\left(\frac{x}{2}\right)^2+\left(\frac{y}{2}\right)^2}= \sqrt{\frac{1}{4}\left(x^2+y^2\right)}=\sqrt{\frac{1}{4}(4)}=1$ . Thus the midpoints lying on the sides determined by vertex $(0,0)$ form a quarter- circle with radius 1.
The set of all midpoints forms a quarter circle at each corner of the square. The area enclosed by all of the midpoints is $4-4\cdot \left(\frac{\pi}{4}\right)=4-\pi \approx .86$ to the nearest hundredth. Thus $100\cdot k=\boxed{86}$
| 86
|
6,403
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_5
| 1
|
Alpha and Beta both took part in a two-day problem-solving competition. At the end of the second day, each had attempted questions worth a total of 500 points. Alpha scored 160 points out of 300 points attempted on the first day, and scored 140 points out of 200 points attempted on the second day. Beta who did not attempt 300 points on the first day, had a positive integer score on each of the two days, and Beta's daily success rate (points scored divided by points attempted) on each day was less than Alpha's on that day. Alpha's two-day success ratio was 300/500 = 3/5. The largest possible two-day success ratio that Beta could achieve is $m/n,$ where $m$ and $n$ are relatively prime positive integers. What is $m+n$
|
Let $q$ be the number of questions Beta takes on day 1 and $a$ be the number he gets right. Let $b$ be the number he gets right on day 2.
These inequalities follow: \[\frac{a}{q} < \frac{160}{300} = \frac{8}{15}\] \[\frac{b}{500-q} < \frac{140}{200} = \frac{7}{10}\] Solving for a and b and adding the two inequalities: \[a + b < \frac{8}{15}q + (350 - \frac{7}{10}q)\] \[a + b < 350 - \frac{1}{6}q\] From here, we see the largest possible value of $a+b$ is $349$
Checking our conditions, we know that $a$ must be positive so therefore $q$ must be positive. A quick check shows that for $2\le q \le 5$ $q$ follows all the conditions and results in $a+b=349$
This makes Beta's success ratio $\frac{349}{500}$ . Thus, the answer is $m+n = 349 + 500 = \boxed{849}$
| 849
|
6,404
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_6
| 1
|
An integer is called snakelike if its decimal representation $a_1a_2a_3\cdots a_k$ satisfies $a_i<a_{i+1}$ if $i$ is odd and $a_i>a_{i+1}$ if $i$ is even . How many snakelike integers between 1000 and 9999 have four distinct digits?
|
We divide the problem into two cases: one in which zero is one of the digits and one in which it is not. In the latter case, suppose we pick digits $x_1,x_2,x_3,x_4$ such that $x_1<x_2<x_3<x_4$ . There are five arrangements of these digits that satisfy the condition of being snakelike: $x_1x_3x_2x_4$ $x_1x_4x_2x_3$ $x_2x_3x_1x_4$ $x_2x_4x_1x_3$ $x_3x_4x_1x_2$ . Thus there are $5\cdot {9\choose 4}=630$ snakelike numbers which do not contain the digit zero.
In the second case we choose zero and three other digits such that $0<x_2<x_3<x_4$ . There are three arrangements of these digits that satisfy the condition of being snakelike: $x_2x_30x_4$ $x_2x_40x_3$ $x_3x_40x_2$ . Because we know that zero is a digit, there are $3\cdot{9\choose 3}=252$ snakelike numbers which contain the digit zero. Thus there are $630+252=\boxed{882}$ snakelike numbers.
| 882
|
6,405
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_6
| 2
|
An integer is called snakelike if its decimal representation $a_1a_2a_3\cdots a_k$ satisfies $a_i<a_{i+1}$ if $i$ is odd and $a_i>a_{i+1}$ if $i$ is even . How many snakelike integers between 1000 and 9999 have four distinct digits?
|
Let's create the snakelike number from digits $a < b < c < d$ , and, if we already picked the digits there are 5 ways to do so, as said in the first solution. And, let's just pick the digits from 0-9. This get's a total count of $5\cdot{10 \choose 4}$ But, this over-counts since it counts numbers like 0213. We can correct for this over-counting. Lock the first digit as 0 and permute 3 other chosen digits $a < b < c$ . There are 2 ways to permute to make the number snakelike, b-a-c, or c-a-b. And, we pick a,b,c from 1 to 9, since 0 has already been chosen as one of the digits. So, the amount we have overcounted by is $2\cdot{9 \choose 3}$ . Thus our answer is $5\cdot{10 \choose 4} - 2\cdot{9 \choose 3} = \boxed{882}$
| 882
|
6,406
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_6
| 3
|
An integer is called snakelike if its decimal representation $a_1a_2a_3\cdots a_k$ satisfies $a_i<a_{i+1}$ if $i$ is odd and $a_i>a_{i+1}$ if $i$ is even . How many snakelike integers between 1000 and 9999 have four distinct digits?
|
We will first decide the order of the 4 digits, greatest to least. To do this, we will pretend that we have selected the digits 1,2,3,4, and we need to arrange them to create a snakelike number. By testing all permutations, there are only 5 ways to make a snakelike number: (1,3,2,4),(1,4,2,3),(2,3,1,4),(2,4,1,3),(3,4,1,2).
Now we select 4 digits to replace the 1,2,3,4.
In first 2 of cases: (1,3,2,4),(1,4,2,3), the leading digit is a 1, which means it is the smallest of our 4 digits. If we select a 0, the leading digit will be a zero, which is bad because all numbers between 1000 and 9999 have nonzero leading digits. So, we need to select our 4 digits only from the pool of 1-9. There are 9 choose 4 ways and there are 2 cases:
$2 \cdot \dbinom{9}{4} = 252$
Thus, there are 252 ways for those 2 cases.
For the next 3 cases, selecting a 0 is okay, so we can select from the pool of 0-9. There are 10 choose 4 ways to select our 4 digits and there are 3 cases:
$3 \cdot \dbinom{10}{4} = 630$
For those 3 cases there are 630 ways.
Thus, our answer is 630+252 = $\boxed{882}$
| 882
|
6,407
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_7
| 1
|
Let $C$ be the coefficient of $x^2$ in the expansion of the product $(1 - x)(1 + 2x)(1 - 3x)\cdots(1 + 14x)(1 - 15x).$ Find $|C|.$
|
Let our polynomial be $P(x)$
It is clear that the coefficient of $x$ in $P(x)$ is $-1 + 2 - 3 + \ldots + 14 - 15 = -8$ , so $P(x) = 1 -8x + Cx^2 + Q(x)$ , where $Q(x)$ is some polynomial divisible by $x^3$
Then $P(-x) = 1 + 8x + Cx^2 + Q(-x)$ and so $P(x)\cdot P(-x) = 1 + (2C - 64)x^2 + R(x)$ , where $R(x)$ is some polynomial divisible by $x^3$
However, we also know $P(x)\cdot P(-x) = (1 - x)(1 + x)(1 +2x)(1 - 2x) \cdots (1 - 15x)(1 + 15x)$ $= (1 - x^2)(1 - 4x^2)\cdots(1 - 225x^2)$ $= 1 - (1 + 4 + \ldots + 225)x^2 + R(x)$
Equating coefficients, we have $2C - 64 = -(1 + 4 + \ldots + 225) = -1240$ , so $-2C = 1176$ and $|C| = \boxed{588}$
| 588
|
6,408
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_7
| 2
|
Let $C$ be the coefficient of $x^2$ in the expansion of the product $(1 - x)(1 + 2x)(1 - 3x)\cdots(1 + 14x)(1 - 15x).$ Find $|C|.$
|
Let $S$ be the set of integers $\{-1,2,-3,\ldots,14,-15\}$ . The coefficient of $x^2$ in the expansion is equal to the sum of the product of each pair of distinct terms, or $C = \sum_{1 \le i \neq j}^{15} S_iS_j$ . Also, we know that \begin{align*}\left(\sum_{i=1}^{n} S_i\right)^2 &= \left(\sum_{i=1}^{n} S_i^2\right) + 2\left(\sum_{1 \le i \neq j}^{15} S_iS_j\right)\\ (-8)^2 &= \frac{15(15+1)(2\cdot 15+1)}{6} + 2C\end{align*} where the left-hand sum can be computed from:
and the right-hand sum comes from the formula for the sum of the first $n$ perfect squares. Therefore, $|C| = \left|\frac{64-1240}{2}\right| = \boxed{588}$
| 588
|
6,409
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_7
| 3
|
Let $C$ be the coefficient of $x^2$ in the expansion of the product $(1 - x)(1 + 2x)(1 - 3x)\cdots(1 + 14x)(1 - 15x).$ Find $|C|.$
|
Let set $N$ be $\{-1, -3, \ldots -15\}$ and set $P$ be $\{2, 4, \ldots 14\}$ . The sum of the negative $x^2$ coefficients is the sum of the products of the elements in all two element sets such that one element is from $N$ and the other is from $P$ . Each summand is a term in the expansion of \[(-1 - 3 - \ldots - 15)(2 + 4 + \ldots + 14)\] which equals $-56 * 64 = -(60^2 - 4^2) = -3584$ . The sum of the positive $x^2$ coefficients is the sum of the products of all two element sets such that the two elements are either both in $N$ or both in $P$ . By counting, the sum is $2992$ , so the sum of all $x^2$ coefficients is $-588$ . Thus, the answer is $\boxed{588}$
| 588
|
6,410
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_7
| 4
|
Let $C$ be the coefficient of $x^2$ in the expansion of the product $(1 - x)(1 + 2x)(1 - 3x)\cdots(1 + 14x)(1 - 15x).$ Find $|C|.$
|
This is just another way of summing the subsets of 2 from $[-1, 2, -3, 4, -5, 6, -7, 8, -9, 10, -11, 12, -13, 14, -15]$ . Start from the right and multiply -15 to everything on its left. Use the distributive property and add all the 14 integers together to get 7. This gives us $-15 * 7$ . Doing this for 14 gives us $14 * -7$ , and for -13 we get $-13 * 6$ . This pattern repeats where every two integers will multiple 7, 6,... to 0. Combining and simplifying the pattern give us this: $-(29 * 7 + 25 * 6 + 21 * 5 + 17 * 4 + 13 * 3 + 9 * 2 + 5*1)$ . The expression gives us -588, or $C = \boxed{588}$ . This is a good solution because it guarantees we never add a product twice, and the pattern is simple to add by hand.
| 588
|
6,411
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_8
| 1
|
Define a regular $n$ -pointed star to be the union of $n$ line segments $P_1P_2, P_2P_3,\ldots, P_nP_1$ such that
There are no regular 3-pointed, 4-pointed, or 6-pointed stars. All regular 5-pointed stars are similar, but there are two non-similar regular 7-pointed stars. How many non-similar regular 1000-pointed stars are there?
|
We use the Principle of Inclusion-Exclusion (PIE).
If we join the adjacent vertices of the regular $n$ -star, we get a regular $n$ -gon. We number the vertices of this $n$ -gon in a counterclockwise direction: $0, 1, 2, 3, \ldots, n-1.$
A regular $n$ -star will be formed if we choose a vertex number $m$ , where $0 \le m \le n-1$ , and then form the line segments by joining the following pairs of vertex numbers: $(0 \mod{n}, m \mod{n}),$ $(m \mod{n}, 2m \mod{n}),$ $(2m \mod{n}, 3m \mod{n}),$ $\cdots,$ $((n-2)m \mod{n}, (n-1)m \mod{n}),$ $((n-1)m \mod{n}, 0 \mod{n}).$
If $\gcd(m,n) > 1$ , then the star degenerates into a regular $\frac{n}{\gcd(m,n)}$ -gon or a (2-vertex) line segment if $\frac{n}{\gcd(m,n)}= 2$ . Therefore, we need to find all $m$ such that $\gcd(m,n) = 1$
Note that $n = 1000 = 2^{3}5^{3}.$
Let $S = \{1,2,3,\ldots, 1000\}$ , and $A_{i}= \{i \in S \mid i\, \textrm{ divides }\,1000\}$ . The number of $m$ 's that are not relatively prime to $1000$ is: $\mid A_{2}\cup A_{5}\mid = \mid A_{2}\mid+\mid A_{5}\mid-\mid A_{2}\cap A_{5}\mid$ $= \left\lfloor \frac{1000}{2}\right\rfloor+\left\lfloor \frac{1000}{5}\right\rfloor-\left\lfloor \frac{1000}{2 \cdot 5}\right\rfloor$ $= 500+200-100 = 600.$
Vertex numbers $1$ and $n-1=999$ must be excluded as values for $m$ since otherwise a regular n-gon, instead of an n-star, is formed.
The cases of a 1st line segment of (0, m) and (0, n-m) give the same star. Therefore we should halve the count to get non-similar stars.
Therefore, the number of non-similar 1000-pointed stars is $\frac{1000-600-2}{2}= \boxed{199}.$
| 199
|
6,412
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_10
| 1
|
circle of radius 1 is randomly placed in a 15-by-36 rectangle $ABCD$ so that the circle lies completely within the rectangle. Given that the probability that the circle will not touch diagonal $AC$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
The location of the center of the circle must be in the $34 \times 13$ rectangle that is one unit away from the sides of rectangle $ABCD$ . We want to find the area of the right triangle with hypotenuse one unit away from $\overline{AC}$ . Let this triangle be $A'B'C'$
Notice that $ABC$ and $A'B'C'$ share the same incenter ; this follows because the corresponding sides are parallel, and so the perpendicular inradii are concurrent, except that the inradii of $\triangle ABC$ extend one unit farther than those of $\triangle A'B'C'$ . From $A = rs$ , we note that $r_{ABC} = \frac{[ABC]}{s_{ABC}} = \frac{15 \cdot 36 /2}{(15+36+39)/2} = 6$ . Thus $r_{A'B'C'} = r_{ABC} - 1 = 5$ , and since the ratio of the areas of two similar figures is equal to the square of the ratio of their corresponding lengths, $[A'B'C'] = [ABC] \cdot \left(\frac{r_{A'B'C'}}{r_{ABC}}\right)^2 = \frac{15 \times 36}{2} \cdot \frac{25}{36} = \frac{375}{2}$
The probability is $\frac{2[A'B'C']}{34 \times 13} = \frac{375}{442}$ , and $m + n = \boxed{817}$
| 817
|
6,413
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_10
| 2
|
circle of radius 1 is randomly placed in a 15-by-36 rectangle $ABCD$ so that the circle lies completely within the rectangle. Given that the probability that the circle will not touch diagonal $AC$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
2004 I AIME-10.png
Let the bisector of $\angle CAD$ be $AE$ , with $E$ on $CD$ . By the angle bisector theorem, $DE = 36/5$ . Since $\triangle AOR \sim \triangle AED$ $O$ is the center of the circle), we find that $AR = 5$ since $OR = 1$ . Also $AT = 35$ so $RT = OQ = 30$
We can apply the same principle again to find that $PT = 27/2$ , and since $QT = 1$ , we find that $PQ = 27/2 - 1 = 25/2$ . The locus of all possible centers of the circle on this "half" of the rectangle is triangle $\triangle OPQ$ . There exists another congruent triangle that is symmetric over $AC$ that has the same area as triangle $\triangle OPQ$ $\triangle APQ$ has area $\frac {1}{2}\cdot OP \cdot PQ = \frac {1}{2}\cdot 30\cdot \frac {25}{2}$ , since $\angle OQP$ is right. Thus the total area that works is $30\cdot \frac {25}{2} = 375$ , and the area of the locus of all centers of any circle with radius 1 is $34\cdot 13 = 442$ . Hence, the desired probability is $\frac {375}{442}$ , and our answer is $\boxed{817}$
| 817
|
6,414
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_10
| 3
|
circle of radius 1 is randomly placed in a 15-by-36 rectangle $ABCD$ so that the circle lies completely within the rectangle. Given that the probability that the circle will not touch diagonal $AC$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
2004 I AIME-10b.png
Again, the location of the center of the circle must be in the $34 \times 13$ rectangle that is one unit away from the sides of rectangle $ABCD$ . We want to find the area of the right triangle with hypotenuse one unit away from $\overline{AC}$
Let $A$ be at the origin, $B (36,0)$ $C (36,15)$ $D (0,15)$ . The slope of $\overline{AC}$ is $\frac{15}{36} = \frac{5}{12}$ . Let $\triangle A'B'C'$ be the right triangle with sides one unit inside $\triangle ABC$ . Since $\overline{AC} || \overline{A'C'}$ , they have the same slope, and the equation of $A'C'$ is $y = \frac{5}{12}x + c$ . Manipulating, $5x - 12y + 12c = 0$ . We need to find the value of $c$ , which can be determined since $\overline{AC}$ is one unit away from $\overline{A'C'}$ . Since the diagonal contains the origin, we can use the distance from a point to the line formula at the origin:
\[\left|\frac{Ax + By + C}{\sqrt{A^2+B^2}}\right| = 1 \Longrightarrow \left|\frac{(5)(0) + (-12)(0) + 12c}{\sqrt{5^2 + (-12)^2}}\right| = 1\] \[c = \pm \frac{13}{12}\]
The two values of $c$ correspond to the triangle on top and below the diagonal. We are considering $A'B'C'$ which is below, so $c = -\frac{13}{12}$ . Then the equation of $\overline{A'C'}$ is $y = \frac{5}{12}x - \frac{13}{12}$ . Solving for its intersections with the lines $y = 1, x = 35$ (boundaries of the internal rectangle), we find the coordinates of $A'B'C'$ are at $A'\ (5,1)\ B'\ (35,1)\ C'\ (35,\frac{27}{2})$ . The area is $\frac{1}{2}bh = \frac{1}{2}(35-5)\left(\frac{27}{2} - 1\right) = \frac{375}{2}$
Finally, the probability is $\frac{2\cdot \mathrm{area\ of\ triangle}}{34 \times 13} = \frac{375}{442}$ , and $m + n = \boxed{817}$
| 817
|
6,415
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_11
| 1
|
solid in the shape of a right circular cone is 4 inches tall and its base has a 3-inch radius. The entire surface of the cone, including its base, is painted. A plane parallel to the base of the cone divides the cone into two solids, a smaller cone-shaped solid $C$ and a frustum -shaped solid $F,$ in such a way that the ratio between the areas of the painted surfaces of $C$ and $F$ and the ratio between the volumes of $C$ and $F$ are both equal to $k$ . Given that $k=\frac m n,$ where $m$ and $n$ are relatively prime positive integers , find $m+n.$
|
Our original solid has volume equal to $V = \frac13 \pi r^2 h = \frac13 \pi 3^2\cdot 4 = 12 \pi$ and has surface area $A = \pi r^2 + \pi r \ell$ , where $\ell$ is the slant height of the cone. Using the Pythagorean Theorem , we get $\ell = 5$ and $A = 24\pi$
Let $x$ denote the radius of the small cone. Let $A_c$ and $A_f$ denote the area of the painted surface on cone $C$ and frustum $F$ , respectively, and let $V_c$ and $V_f$ denote the volume of cone $C$ and frustum $F$ , respectively. Because the plane cut is parallel to the base of our solid, $C$ is similar to the uncut solid and so the height and slant height of cone $C$ are $\frac{4}{3}x$ and $\frac{5}{3}x$ , respectively. Using the formula for lateral surface area of a cone, we find that $A_c=\frac{1}{2}c\cdot \ell=\frac{1}{2}(2\pi x)\left(\frac{5}{3}x\right)=\frac{5}{3}\pi x^2$ . By subtracting $A_c$ from the surface area of the original solid, we find that $A_f=24\pi - \frac{5}{3}\pi x^2$
Next, we can calculate $V_c=\frac{1}{3}\pi r^2h=\frac{1}{3}\pi x^2 \left(\frac{4}{3}x\right)=\frac{4}{9}\pi x^3$ . Finally, we subtract $V_c$ from the volume of the original cone to find that $V_f=12\pi - \frac{4}{9}\pi x^3$ . We know that $\frac{A_c}{A_f}=\frac{V_c}{V_f}=k.$ Plugging in our values for $A_c$ $A_f$ $V_c$ , and $V_f$ , we obtain the equation $\frac{\frac{5}{3}\pi x^2}{24\pi - \frac{5}{3}\pi x^2}=\frac{\frac{4}{9}\pi x^3}{12\pi - \frac{4}{9}\pi x^3}$ . We can take reciprocals of both sides to simplify this equation to $\frac{72}{5x^2} - 1 = \frac{27}{x^3} - 1$ and so $x = \frac{15}{8}$ . Then $k = \frac{\frac{5}{3}\pi x^2}{24\pi - \frac{5}{3}\pi x^2}= \frac{125}{387} = \frac mn$ so the answer is $m+n=125+387=\boxed{512}$
| 512
|
6,416
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_11
| 2
|
solid in the shape of a right circular cone is 4 inches tall and its base has a 3-inch radius. The entire surface of the cone, including its base, is painted. A plane parallel to the base of the cone divides the cone into two solids, a smaller cone-shaped solid $C$ and a frustum -shaped solid $F,$ in such a way that the ratio between the areas of the painted surfaces of $C$ and $F$ and the ratio between the volumes of $C$ and $F$ are both equal to $k$ . Given that $k=\frac m n,$ where $m$ and $n$ are relatively prime positive integers , find $m+n.$
|
Our original solid $V$ has surface area $A_v = \pi r^2 + \pi r \ell$ , where $\ell$ is the slant height of the cone. Using the Pythagorean Theorem or Pythagorean Triple knowledge, we obtain $\ell = 5$ and lateral area $A_\ell = 15\pi$ . The area of the base is $A_B = 3^2\pi = 9\pi$
$V$ and $C$ are similar cones, because the plane that cut out $C$ was parallel to the base of $V$ . Let $x$ be the scale factor between the original cone and the small cone $C$ in one dimension. Because the scale factor is uniform in all dimensions, $x^2$ relates corresponding areas of $C$ and $V$ , and $x^3$ relates corresponding volumes. Then, the ratio of the painted areas $\frac{A_c}{A_f}$ is $\frac{15\pi x^2}{9\pi + 15\pi - 15\pi x^2} = \frac{5 x^2}{8 - 5 x^2} = k$ and the ratio of the volumes $\frac{V_c}{V_f}$ is $\frac{x^3}{1 - x^3} = k$ . Since both ratios are equal to $k$ , they are equal to each other. Therefore, $\frac{5 x^2}{8 - 5 x^2} = \frac{x^3}{1 - x^3}$
Now we must merely solve for x and substitute back into either ratio. Cross multiplying gives $5 x^2(1 - x^3) = x^3(8 - 5 x^2)$ . Dividing both sides by $x^2$ and distributing the $x$ on the right, we have $5 - 5 x^3 = 8 x - 5 x^3$ , and so $8 x = 5$ and $x = \frac{5}{8}$ . Substituting back into the easier ratio, we have $\frac{(\frac{5}{8})^3}{1 - (\frac{5}{8})^3} = \frac{\frac{125}{512}}{\frac{387}{512}} = \frac{125}{387}$ . And so we have $m + n = 125 + 387 = \boxed{512}$
| 512
|
6,417
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_13
| 1
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The polynomial $P(x)=(1+x+x^2+\cdots+x^{17})^2-x^{17}$ has $34$ complex roots of the form $z_k = r_k[\cos(2\pi a_k)+i\sin(2\pi a_k)], k=1, 2, 3,\ldots, 34,$ with $0 < a_1 \le a_2 \le a_3 \le \cdots \le a_{34} < 1$ and $r_k>0.$ Given that $a_1 + a_2 + a_3 + a_4 + a_5 = m/n,$ where $m$ and $n$ are relatively prime positive integers, find $m+n.$
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We see that the expression for the polynomial $P$ is very difficult to work with directly, but there is one obvious transformation to make: sum the geometric series
\begin{align*} P(x) &= \left(\frac{x^{18} - 1}{x - 1}\right)^2 - x^{17} = \frac{x^{36} - 2x^{18} + 1}{x^2 - 2x + 1} - x^{17}\\ &= \frac{x^{36} - x^{19} - x^{17} + 1}{(x - 1)^2} = \frac{(x^{19} - 1)(x^{17} - 1)}{(x - 1)^2} \end{align*}
This expression has roots at every $17$ th root and $19$ th roots of unity , other than $1$ . Since $17$ and $19$ are relatively prime , this means there are no duplicate roots. Thus, $a_1, a_2, a_3, a_4$ and $a_5$ are the five smallest fractions of the form $\frac m{19}$ or $\frac n {17}$ for $m, n > 0$
$\frac 3 {17}$ and $\frac 4{19}$ can both be seen to be larger than any of $\frac1{19}, \frac2{19}, \frac3{19}, \frac 1{17}, \frac2{17}$ , so these latter five are the numbers we want to add.
$\frac1{19}+ \frac2{19}+ \frac3{19}+ \frac 1{17}+ \frac2{17}= \frac6{19} + \frac 3{17} = \frac{6\cdot17 + 3\cdot19}{17\cdot19} = \frac{159}{323}$ and so the answer is $159 + 323 = \boxed{482}$
| 482
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6,418
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_14
| 1
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A unicorn is tethered by a $20$ -foot silver rope to the base of a magician's cylindrical tower whose radius is $8$ feet. The rope is attached to the tower at ground level and to the unicorn at a height of $4$ feet. The unicorn has pulled the rope taut, the end of the rope is $4$ feet from the nearest point on the tower, and the length of the rope that is touching the tower is $\frac{a-\sqrt{b}}c$ feet, where $a, b,$ and $c$ are positive integers , and $c$ is prime. Find $a+b+c.$
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Looking from an overhead view, call the center of the circle $O$ , the tether point to the unicorn $A$ and the last point where the rope touches the tower $B$ $\triangle OAB$ is a right triangle because $OB$ is a radius and $BA$ is a tangent line at point $B$ . We use the Pythagorean Theorem to find the horizontal component of $AB$ has length $4\sqrt{5}$
Now look at a side view and "unroll" the cylinder to be a flat surface. Let $C$ be the bottom tether of the rope, let $D$ be the point on the ground below $A$ , and let $E$ be the point directly below $B$ Triangles $\triangle CDA$ and $\triangle CEB$ are similar right triangles . By the Pythagorean Theorem $CD=8\cdot\sqrt{6}$
Let $x$ be the length of $CB$ \[\frac{CA}{CD}=\frac{CB}{CE}\implies \frac{20}{8\sqrt{6}}=\frac{x}{8\sqrt{6}-4\sqrt{5}}\implies x=\frac{60-\sqrt{750}}{3}\]
Therefore $a=60, b=750, c=3, a+b+c=\boxed{813}$
| 813
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6,419
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_14
| 2
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A unicorn is tethered by a $20$ -foot silver rope to the base of a magician's cylindrical tower whose radius is $8$ feet. The rope is attached to the tower at ground level and to the unicorn at a height of $4$ feet. The unicorn has pulled the rope taut, the end of the rope is $4$ feet from the nearest point on the tower, and the length of the rope that is touching the tower is $\frac{a-\sqrt{b}}c$ feet, where $a, b,$ and $c$ are positive integers , and $c$ is prime. Find $a+b+c.$
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Note that by Power of a Point, the point the unicorn is at has power $4 \cdot 20 = 80$ which implies that the tangent from that point to the tower is of length $\sqrt{80}=4\sqrt{5},$ however this is length of the rope projected into 2-D. If we let $\theta$ be the angle between the horizontal and the rope, we have that $\cos\theta=\frac{1}{5}$ which implies that $\sin\theta=\frac{2\sqrt{6}}{5}.$ Note that the portion of rope not on the tower is $4\sqrt{5} \cdot \frac{5}{2\sqrt{6}}= \frac{5\sqrt{30}}{3},$ the requested length of rope is $20-\frac{5\sqrt{30}}{3}=\frac{60-\sqrt{750}}{3}$ thus the requested sum is $\boxed{813}.$
| 813
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6,420
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_I_Problems/Problem_15
| 1
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For all positive integers $x$ , let \[f(x)=\begin{cases}1 & \text{if }x = 1\\ \frac x{10} & \text{if }x\text{ is divisible by 10}\\ x+1 & \text{otherwise}\end{cases}\] and define a sequence as follows: $x_1=x$ and $x_{n+1}=f(x_n)$ for all positive integers $n$ . Let $d(x)$ be the smallest $n$ such that $x_n=1$ . (For example, $d(100)=3$ and $d(87)=7$ .) Let $m$ be the number of positive integers $x$ such that $d(x)=20$ . Find the sum of the distinct prime factors of $m$
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We backcount the number of ways. Namely, we start at $x_{20} = 1$ , which can only be reached if $x_{19} = 10$ , and then we perform $18$ operations that either consist of $A: (-1)$ or $B: (\times 10)$ . We represent these operations in a string format, starting with the operation that sends $f(x_{18}) = x_{19}$ and so forth downwards. There are $2^9$ ways to pick the first $9$ operations; however, not all $9$ of them may be $A: (-1)$ otherwise we return back to $x_{10} = 1$ , contradicting our assumption that $20$ was the smallest value of $n$ . Using complementary counting , we see that there are only $2^9 - 1$ ways.
Since we performed the operation $B: (\times 10)$ at least once in the first $9$ operations, it follows that $x_{10} \ge 20$ , so that we no longer have to worry about reaching $1$ again.
However, we must also account for a sequence of $10$ or more $A: (-1)$ s in a row, because that implies that at least one of those numbers was divisible by $10$ , yet the $\frac{x}{10}$ was never used, contradiction. We must use complementary counting again by determining the number of strings of $A,B$ s of length $18$ such that there are $10$ $A$ s in a row. The first ten are not included since we already accounted for that scenario above, so our string of $10$ $A$ s must be preceded by a $B$ . There are no other restrictions on the remaining seven characters. Letting $\square$ to denote either of the functions, and $^{[k]}$ to indicate that the character appears $k$ times in a row, our bad strings can take the forms: \begin{align*} &\underbrace{BA^{[10]}}\square \square \square \square \square \square \square\\ &\square \underbrace{BA^{[10]}}\square \square \square \square \square \square \\ &\qquad \quad \cdots \quad \cdots \\ &\square \square \square \square \square \square \underbrace{BA^{[10]}} \square \\ &\square \square \square \square \square \square \square \underbrace{BA^{[10]}} \\ \end{align*}
There are $2^7$ ways to select the operations for the $7$ $\square$ s, and $8$ places to place our $BA^{[10]}$ block. Thus, our answer is $2^9(2^9-1)-8\cdot 2^7 = 2^{18} - 2^9 - 8 \cdot 2^7 = 2^9 \times 509$ , and the answer is $\boxed{511}$
| 511
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6,421
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_1
| 1
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chord of a circle is perpendicular to a radius at the midpoint of the radius. The ratio of the area of the larger of the two regions into which the chord divides the circle to the smaller can be expressed in the form $\frac{a\pi+b\sqrt{c}}{d\pi-e\sqrt{f}},$ where $a, b, c, d, e,$ and $f$ are positive integers $a$ and $e$ are relatively prime , and neither $c$ nor $f$ is divisible by the square of any prime . Find the remainder when the product $abcdef$ is divided by 1000.
2004 AIME II Problem 1.png
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Let $r$ be the length of the radius of the circle. A right triangle is formed by half of the chord, half of the radius (since the chord bisects it), and the radius. Thus, it is a $30^\circ$ $60^\circ$ $90^\circ$ triangle , and the area of two such triangles is $2 \cdot \frac{1}{2} \cdot \frac{r}{2} \cdot \frac{r\sqrt{3}}{2} = \frac{r^2\sqrt{3}}{4}$ . The central angle which contains the entire chord is $60 \cdot 2 = 120$ degrees , so the area of the sector is $\frac{1}{3}r^2\pi$ ; the rest of the area of the circle is then equal to $\frac{2}{3}r^2\pi$
The smaller area cut off by the chord is equal to the area of the sector minus the area of the triangle. The larger area is equal to the area of the circle not within the sector and the area of the triangle. Thus, the desired ratio is $\frac{\frac{2}{3}r^2\pi + \frac{r^2\sqrt{3}}{4}}{\frac{1}{3}r^2\pi - \frac{r^2\sqrt{3}}{4}} = \frac{8\pi + 3\sqrt{3}}{4\pi - 3\sqrt{3}}$
Therefore, $abcdef = 2^53^4 = 2592 \Longrightarrow \boxed{592}$
| 592
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6,422
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_2
| 1
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A jar has $10$ red candies and $10$ blue candies. Terry picks two candies at random, then Mary picks two of the remaining candies at random. Given that the probability that they get the same color combination, irrespective of order, is $m/n,$ where $m$ and $n$ are relatively prime positive integers , find $m+n.$
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The probability that Terry picks two red candies is $\frac{10 \cdot 9}{20 \cdot 19} = \frac{9}{38}$ , and the probability that Mary picks two red candies after Terry chooses two red candies is $\frac{7\cdot8}{18\cdot17} = \frac{28}{153}$ . So the probability that they both pick two red candies is $\frac{9}{38} \cdot \frac{28}{153} = \frac{14}{323}$ . The same calculation works for the blue candies.
The probability that Terry picks two different candies is $\frac{20\cdot10}{20\cdot19} = \frac{10}{19}$ , and the probability that Mary picks two different candies after Terry picks two different candies is $\frac{18\cdot 9}{18\cdot 17} = \frac{9}{17}$ . Thus, the probability that they both choose two different candies is $\frac{10}{19}\cdot\frac{9}{17} = \frac{90}{323}$ . Then the total probability is
\[2 \cdot \frac{14}{323} + \frac{90}{323} = \frac{118}{323}\]
and so the answer is $118 + 323 = \boxed{441}$
| 441
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6,423
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_3
| 1
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A solid rectangular block is formed by gluing together $N$ congruent 1-cm cubes face to face. When the block is viewed so that three of its faces are visible, exactly $231$ of the 1-cm cubes cannot be seen. Find the smallest possible value of $N.$
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The $231$ cubes which are not visible must lie below exactly one layer of cubes. Thus, they form a rectangular solid which is one unit shorter in each dimension. If the original block has dimensions $l \times m \times n$ , we must have $(l - 1)\times(m-1) \times(n - 1) = 231$ . The prime factorization of $231 = 3\cdot7\cdot11$ , so we have a variety of possibilities; for instance, $l - 1 = 1$ and $m - 1 = 11$ and $n - 1 = 3 \cdot 7$ , among others. However, it should be fairly clear that the way to minimize $l\cdot m\cdot n$ is to make $l$ and $m$ and $n$ as close together as possible, which occurs when the smaller block is $3 \times 7 \times 11$ . Then the extra layer makes the entire block $4\times8\times12$ , and $N= \boxed{384}$
| 384
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6,424
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_4
| 1
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How many positive integers less than 10,000 have at most two different digits
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First, let's count numbers with only a single digit. We have nine of these for each length, and four lengths, so 36 total numbers.
Now, let's count those with two distinct digits. We handle the cases "0 included" and "0 not included" separately.
There are ${9 \choose 2}$ ways to choose two digits, $A$ and $B$ . Given two digits, there are $2^n - 2$ ways to arrange them in an $n$ -digit number, for a total of $(2^1 - 2) + (2^2 - 2) + (2^3 -2) + (2^4 - 2) = 22$ such numbers (or we can list them: $AB, BA, AAB, ABA, BAA, ABB, BAB, BBA, AAAB, AABA, ABAA,$ $BAAA, AABB, ABAB, BAAB, ABBA, BABA, BBAA, ABBB, BABB, BBAB, BBBA$ ). Thus, we have ${9 \choose 2} \cdot 22 = 36\cdot22 = 792$ numbers of this form.
Now, suppose 0 is one of our digits. We have nine choices for the other digit. For each choice, we have $2^{n - 1} - 1$ $n$ -digit numbers we can form, for a total of $(2^0 - 1) + (2^1 - 1) + (2^2 - 1) + (2^3 - 1) = 11$ such numbers (or we can list them: $A0, A00, A0A, AA0, A000, AA00, A0A0, A00A, AAA0, AA0A, A0AA$ ). This gives us $9\cdot 11 = 99$ numbers of this form.
Thus, in total, we have $36 + 792 + 99 = \boxed{927}$ such numbers.
| 927
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6,425
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_4
| 2
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How many positive integers less than 10,000 have at most two different digits
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We use casework on the number of digits for this problem.
If the number has a single digit, namely the number $n \in [1,9],$ we can clearly all such $n$ work.
If the number has two digits, or the number $n \in [10,99]$ we can clearly see all such $n$ work.
If the number $n$ has three digits, there are a total of $900$ three digit numbers, and there are $9 \cdot 9 \cdot 8$ numbers that have all distinct digits so there are $900 - 9 \cdot 9 \cdot 8$ total three digit numbers that work.
If the number $n$ has four digits, then we have a total of $9 +\binom{9}{2}\left(\binom{4}{1}+\binom{4}{2} +\binom{4}{3}\right) + 9 \cdot \left(\binom{3}{1}+\binom{3}{2} +\binom{3}{3}\right)$ so in total we get $\boxed{927}$ numbers that work.
| 927
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6,426
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_5
| 1
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In order to complete a large job, $1000$ workers were hired, just enough to complete the job on schedule. All the workers stayed on the job while the first quarter of the work was done, so the first quarter of the work was completed on schedule. Then $100$ workers were laid off, so the second quarter of the work was completed behind schedule. Then an additional $100$ workers were laid off, so the third quarter of the work was completed still further behind schedule. Given that all workers work at the same rate, what is the minimum number of additional workers, beyond the $800$ workers still on the job at the end of the third quarter, that must be hired after three-quarters of the work has been completed so that the entire project can be completed on schedule or before?
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A train is traveling at $1000$ miles per hour and has one hour to reach its destination $1000$ miles away. After $15$ minutes and $250$ miles it slows to $900$ mph, and thus takes $\frac{250}{900}(60)=\frac{50}{3}$ minutes to travel the next $250$ miles. Then it slows to $800$ mph, so the next quarter takes $\frac{250}{800}(60)=\frac{150}{8}$ . The train then has $60-15-\frac{50}{3}-\frac{150}{8}=230/24$ minutes left to travel 250 miles, and doing the arithmetic shows that during this last stretch it must travel more than $1565$ mph to make it on time. Therefore the company needs to add $1566-800 = \boxed{766}$ more workers.
| 766
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6,427
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_5
| 2
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In order to complete a large job, $1000$ workers were hired, just enough to complete the job on schedule. All the workers stayed on the job while the first quarter of the work was done, so the first quarter of the work was completed on schedule. Then $100$ workers were laid off, so the second quarter of the work was completed behind schedule. Then an additional $100$ workers were laid off, so the third quarter of the work was completed still further behind schedule. Given that all workers work at the same rate, what is the minimum number of additional workers, beyond the $800$ workers still on the job at the end of the third quarter, that must be hired after three-quarters of the work has been completed so that the entire project can be completed on schedule or before?
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Let each worker's speed be $w$ , the entire time be $t$ , and the total work be $1$
From the initial problem statement, we have $1000w\cdot\frac{1}{4}t=\frac{1}{4}$
We need to find the time the next quarter takes to complete the same amount of work, which is $\frac{1000}{900}\cdot\frac{1}{4}\cdot t=\frac{5}{18}t$
Similarly, we find that the time the third quarter takes is $\frac{1000}{800}\cdot\frac{1}{4}\cdot t=\frac{5}{16}t$
Finally, the time the last quarter takes is $t\left[1-\left(\frac{1}{4}+\frac{5}{18}+\frac{5}{16}\right)\right]=\frac{23}{144}t$
We then let the number of workers needed be $x$ , so we have the equation $\left(800+x\right)w\cdot\frac{23}{144}t=\frac{1}{4}$ . Dividing by the first equation, we have
\[\frac{800+x}{1000}\cdot\frac{\frac{23}{144}}{\frac{1}{4}}=1\] \[\frac{800+x}{1000}\cdot\frac{23}{36}=1\] \[800+x=\frac{36000}{23}\] \[x=\frac{17600}{23}.\]
We can't have a part of a worker, so we take the ceiling of $x$ , which we find to be $\boxed{766}$
| 766
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6,428
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_5
| 3
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In order to complete a large job, $1000$ workers were hired, just enough to complete the job on schedule. All the workers stayed on the job while the first quarter of the work was done, so the first quarter of the work was completed on schedule. Then $100$ workers were laid off, so the second quarter of the work was completed behind schedule. Then an additional $100$ workers were laid off, so the third quarter of the work was completed still further behind schedule. Given that all workers work at the same rate, what is the minimum number of additional workers, beyond the $800$ workers still on the job at the end of the third quarter, that must be hired after three-quarters of the work has been completed so that the entire project can be completed on schedule or before?
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Suppose $1000$ workers can complete one quarter of the job in one day. After the first day, there were $900$ workers remaining so the second quarter was completed in $\frac{10}{9}$ days. Now there are only $800$ workers remaining so the third quarter can be completed in $\frac{10}{8}$ days. It has been $1+\frac{10}{9}+\frac{10}{8}=\frac{121}{36}$ days since the job started, and we still have $\frac{23}{36}$ days to complete the last quarter of the job, so we need $\frac{36}{23}$ of $1000$ workers, or $1565.2173913$ . Since we can’t hire fractional workers, $1566-800=\boxed{766}$ must be the answer.
| 766
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6,429
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_6
| 1
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Three clever monkeys divide a pile of bananas. The first monkey takes some bananas from the pile, keeps three-fourths of them, and divides the rest equally between the other two. The second monkey takes some bananas from the pile, keeps one-fourth of them, and divides the rest equally between the other two. The third monkey takes the remaining bananas from the pile, keeps one-twelfth of them, and divides the rest equally between the other two. Given that each monkey receives a whole number of bananas whenever the bananas are divided, and the numbers of bananas the first, second, and third monkeys have at the end of the process are in the ratio $3: 2: 1,$ what is the least possible total for the number of bananas?
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Denote the number of bananas the first monkey took from the pile as $b_1$ , the second $b_2$ , and the third $b_3$ ; the total is $b_1 + b_2 + b_3$ . Thus, the first monkey got $\frac{3}{4}b_1 + \frac{3}{8}b_2 + \frac{11}{24}b_3$ , the second monkey got $\frac{1}{8}b_1 + \frac{1}{4}b_2 + \frac{11}{24}b_3$ , and the third monkey got $\frac{1}{8}b_1 + \frac{3}{8}b_2 + \frac{1}{12}b_3$
Taking into account the ratio aspect, say that the third monkey took $x$ bananas in total. Then,
$x = \frac{1}{4}b_1 + \frac{1}{8}b_2 + \frac{11}{72}b_3 = \frac{1}{16}b_1 + \frac{1}{8}b_2 + \frac{11}{48}b_3 = \frac{1}{8}b_1 + \frac{3}{8}b_2 + \frac{1}{12}b_3$
Solve this to find that $\frac{b_1}{11} = \frac{b_2}{13} = \frac{b_3}{27}$ . All three fractions must be integral. Also note some other conditions we have picked up in the course of the problem, namely that $b_1$ is divisible by $8$ $b_2$ is divisible by $8$ , and $b_3$ is divisible by $72$ (however, since the denominator contains a $27$ , the factors of $3$ cancel, and it only really needs to be divisible by $8$ ). Thus, the minimal value is when each fraction is equal to $8$ , and the solution is $8(11 + 13 + 27) = \boxed{408}$
| 408
|
6,430
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_6
| 2
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Three clever monkeys divide a pile of bananas. The first monkey takes some bananas from the pile, keeps three-fourths of them, and divides the rest equally between the other two. The second monkey takes some bananas from the pile, keeps one-fourth of them, and divides the rest equally between the other two. The third monkey takes the remaining bananas from the pile, keeps one-twelfth of them, and divides the rest equally between the other two. Given that each monkey receives a whole number of bananas whenever the bananas are divided, and the numbers of bananas the first, second, and third monkeys have at the end of the process are in the ratio $3: 2: 1,$ what is the least possible total for the number of bananas?
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Let the first monkey take $8x$ bananas, the second monkey take $8y$ , and the third monkey take $24z$ . I chose these numbers to make it so, when each monkey splits his bananas, they will get an integer amount of each variable. So, when the first monkey distributes his bananas, he gets $6x$ , and the other monkeys get $1x$ . So, we can make expressions for how many bananas each monkey gets. Also, the variables have to be integers too.
Monkey 1: $6x+3y+11z$
Monkey 2: $1x+2y+11z$
Monkey 3: $1x+3y+2z$
So, they are in a ratio $3:2:1$ . But, we can turn it into an equation by multiplying the amount of bananas each monkey has by 2, 3, 6. Now, the ratio is $6:6:6$ , so, $12x+6y+22z = 3x+6y+33z = 6x+18y+12z$ . Subtracting $3x+6y+12z$ from all, we get $9x+10z = 21z = 3x+12y$ . Let's split this into 3 equations.
\[9x+10z = 21z\] \[21z = 3x+12y\] \[9x+10z = 3x+12y\]
Let's look at the first equation. Rearranging, it gets us $9x = 11z$
We can rearrange the third equation, then divide by 2, then subtract the second equation. \[21z-6y = 12y-5z\] \[26z = 18y\] \[13z = 9y\]
It is clear $z$ is a multiple of 9, so let $z = 9$ . Then we get the $x = 11$ , and $y = 13$ . Testing, we confirm this will get the first monkey 204 bananas, the second 136, and the third, 68. Adding them up, we get that there were $\boxed{408}$ bananas originally in the pile.
| 408
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6,431
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_6
| 3
|
Three clever monkeys divide a pile of bananas. The first monkey takes some bananas from the pile, keeps three-fourths of them, and divides the rest equally between the other two. The second monkey takes some bananas from the pile, keeps one-fourth of them, and divides the rest equally between the other two. The third monkey takes the remaining bananas from the pile, keeps one-twelfth of them, and divides the rest equally between the other two. Given that each monkey receives a whole number of bananas whenever the bananas are divided, and the numbers of bananas the first, second, and third monkeys have at the end of the process are in the ratio $3: 2: 1,$ what is the least possible total for the number of bananas?
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In this solution, you start out the same as solution one. Convert everything into the fractions of largest denominator terms (this is necessary) until you get
$27x=11z$ $27y=13z$
While solving, make sure to leave a list of numbers that must divide $x$ $y$ , and $z$ . For example, just by looking at the basic fractions you receive from writing the starting equations, 24 divides $z$ , 8 divides $y$ and $x$ . In the expression above, it's also clear that 27 divides $z$ , 13 divides $y$ , and 11 divides $z$ . You might be wondering why I wrote the expression in terms of z. That's because $z$ has the largest divisor. The LCM of all it's divisors shows that $z$ must be divisible by 216. The total amount of bananas can be found to equal to $17z/9$ . This means there are two possible solutions under 1000: 408 and 816. Trial and error can be done quickly to find the smallest possible solution to be $\boxed{408}$
| 408
|
6,432
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_6
| 4
|
Three clever monkeys divide a pile of bananas. The first monkey takes some bananas from the pile, keeps three-fourths of them, and divides the rest equally between the other two. The second monkey takes some bananas from the pile, keeps one-fourth of them, and divides the rest equally between the other two. The third monkey takes the remaining bananas from the pile, keeps one-twelfth of them, and divides the rest equally between the other two. Given that each monkey receives a whole number of bananas whenever the bananas are divided, and the numbers of bananas the first, second, and third monkeys have at the end of the process are in the ratio $3: 2: 1,$ what is the least possible total for the number of bananas?
|
Let $A,B,C$ be the fraction of bananas taken by the first, second, and third monkeys respectively. Then we have the system of equations \[\frac{3}{4}A+\frac{3}{8}B+\frac{11}{24}C=\frac{1}{2}\] \[\frac{1}{8}A+\frac{1}{4}B+\frac{11}{24}C=\frac{1}{3}\] \[\frac{1}{8}A+\frac{3}{8}B+\frac{2}{24}C=\frac{1}{6}.\] Solve this your favorite way to get that \[(A,B,C)=\left( \frac{11}{51}, \frac{13}{51}, \frac{9}{17} \right).\] We need the amount taken by the first and second monkeys to be divisible by 8 and the third by 24 (but for the third, we already have divisibility by 9). Thus our minimum is $8 \cdot 51 = \boxed{408}.$
| 408
|
6,433
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 1
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Since $EF$ is the perpendicular bisector of $\overline{BB'}$ , it follows that $BE = B'E$ (by SAS). By the Pythagorean Theorem , we have $AB' = 15$ . Similarly, from $BF = B'F$ , we have \begin{align*} BC^2 + CF^2 = B'D^2 + DF^2 &\Longrightarrow BC^2 + 9 = (BC - 15)^2 + 484 \\ BC &= \frac{70}{3} \end{align*} Thus the perimeter of $ABCD$ is $2\left(25 + \frac{70}{3}\right) = \frac{290}{3}$ , and our answer is $m+n=\boxed{293}$
| 293
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6,434
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 2
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Let $A = (0,0), B=(0,25)$ , so $E = (0,8)$ and $F = (l,22)$ , and let $l = AD$ be the length of the rectangle. The slope of $EF$ is $\frac{14}{l}$ and so the equation of $EF$ is $y -8 = \frac{14}{l}x$ . We know that $EF$ is perpendicular to and bisects $BB'$ . The slope of $BB'$ is thus $\frac{-l}{14}$ , and so the equation of $BB'$ is $y -25 = \frac{-l}{14}x$ . Let the point of intersection of $EF, BB'$ be $G$ . Then the y-coordinate of $G$ is $\frac{25}{2}$ , so \begin{align*} \frac{14}{l}x &= y-8 = \frac{9}{2}\\ \frac{-l}{14}x &= y-25 = -\frac{25}{2}\\ \end{align*} Dividing the two equations yields
The answer is $\boxed{293}$ as above.
| 293
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6,435
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 3
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Firstly, observe that if we are given that $AE=8$ and $BE=17$ , the length of the triangle is given and the height depends solely on the length of $CF$ . Let Point $A = (0,0)$ . Since $AE=8$ , point E is at (8,0). Next, point $B$ is at $(25,0)$ since $BE=17$ and point $B'$ is at $(0,-15)$ since $BE=AE$ by symmetry. Draw line segment $BB'$ . Notice that this is perpendicular to $EF$ by symmetry. Next, find the slope of EB, which is $\frac{15}{25}=\frac{3}{5}$ . Then, the slope of $EF$ is - $\frac{5}{3}$
Line EF can be written as y= $-\frac{5}{3}x+b$ . Plug in the point $(8,0)$ , and we get the equation of EF to be y= $_\frac{5}{3}x+\frac{40}{3}$ . Since the length of $AB$ =25, a point on line $EF$ lies on $DC$ when $x=25-3=22$ . Plug in $x=22$ into our equation to get $y=-\frac{70}{3}$ $|y|=BC=\frac{70}{3}$ . Therefore, our answer is $2(AB+BC)=2\left(25+\frac{70}{3}\right)=2\left(\frac{145}{3}\right)=\frac{290}{3}= \boxed{293}$
| 293
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6,436
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 4
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Firstly, note that $B'E=BE=17$ , so $AB'=\sqrt{17^2-8^2}=15$ . Then let $\angle BEF=\angle B'EF=\theta$ , so $\angle B'EA = \pi-2\theta$ . Then $\tan(\pi-2\theta)=\frac{15}{8}$ , or
\[\frac{2\tan(\theta)}{\tan^2(\theta)-1}=\frac{15}{8}\] using supplementary and double angle identities. Multiplying though and factoring yields
\[(3\tan(\theta)-5)(5\tan(\theta)+3)=0\]
It is clear from the problem setup that $0<\theta<\frac\pi2$ , so the correct value is $\tan(\theta)=\frac53$ . Next, extend rays $\overrightarrow{BC}$ and $\overrightarrow{EF}$ to intersect at $C'$ . Then $\tan(\theta)=\frac{BC'}{17}=\frac53$ , so $BC'=\frac{85}{3}$ . By similar triangles, $CC'=\frac{3}{17}BC'=\frac{15}{3}$ , so $BC=\frac{70}{3}$ . The perimeter is $\frac{140}{3}+50=\frac{290}{3}\Longrightarrow \boxed{293}$
| 293
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6,437
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 5
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Use the prepared diagram for this solution.
Call the intersection of DF and B'C' G. AB'E is an 8-15-17 right triangle, and so are B'DG and C'FG. Since C'F is 3, then using the properties of similar triangles GF is 51/8. DF is 22, so DG is 125/8. Finally, DB can to calculated to be 25/3.
Add all the sides together to get $\boxed{293}$
| 293
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6,438
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 6
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Call the intersection of $B'C'$ $BC$ , and $EF$ $G$ . Since $FCBE$ and $FC'B'E$ are congruent, we know that the three lines intersect.
We already know $AB$ so we just need to find $CB$ , call it $x$ . Drop an altitude from $F$ to $AB$ and call it $H$ $EH=EB-FC=14$ . Using Pythagorean Theorem, we have $EF=\sqrt{x^2+14^2}$ . Triangles $EFH$ and $EGB$ are similar (AA), so we get \[\frac{HF}{BG}=\frac{EH}{EB}\] \[\frac{x}{x+GC}=\frac{14}{17}\] Simplify and we get $GC=\frac{3x}{14}$
We find the area of $FCBE$ by using the fact that it is a trapezoid. $[FCBE]=\frac{(3+17)x}{2}=10x$
A different way to find the area: $[FCBE]=\frac{1}{2} EG\cdot($ height of $EGB$ with $EG$ as base $)-[FGC]$
Since $GBE$ and $G'B'E$ are congruent(SAS), their height from $EG$ is the same. $B'B=\sqrt{AB'^2+AB^2}=5\sqrt{34}$ $EG=\sqrt{EB^2+BG^2}=\sqrt{(\frac{17x}{14})^2+17^2}=17\sqrt{\frac{x^2}{196}+1}$
\[[FCBE]=\frac{1}{2} \cdot 17 \cdot \sqrt{\frac{x^2}{196}+1} \cdot \frac{5\sqrt{34}}{2}-\frac{9x}{28}\] \[280x+9x=7\cdot 5 \cdot \sqrt{34} \cdot 17 \cdot \sqrt{\frac{x^2}{196}+1}\] \[17^4 x^2=49 \cdot 25 \cdot 34 \cdot 17^2 \cdot (\frac{x^2}{196}+1)\] \[17x^2=\frac{25}{2}x^2+2450\] \[x=\frac{70}{3}\]
The perimeter is $\frac{140}{3}+50=\frac{290}{3},$ so our answer is $\boxed{293}$
| 293
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6,439
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_7
| 7
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$ABCD$ is a rectangular sheet of paper that has been folded so that corner $B$ is matched with point $B'$ on edge $AD.$ The crease is $EF,$ where $E$ is on $AB$ and $F$ is on $CD.$ The dimensions $AE=8, BE=17,$ and $CF=3$ are given. The perimeter of rectangle $ABCD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m+n.$
[asy] size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(25,0), C=(25,70/3), D=(0,70/3), E=(8,0), F=(22,70/3), Bp=reflect(E,F)*B, Cp=reflect(E,F)*C; draw(F--D--A--E); draw(E--B--C--F, linetype("4 4")); filldraw(E--F--Cp--Bp--cycle, white, black); pair point=( 12.5, 35/3 ); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$D$", D, dir(point--D)); label("$E$", E, dir(point--E)); label("$F$", F, dir(point--F)); label("$B^\prime$", Bp, dir(point--Bp)); label("$C^\prime$", Cp, dir(point--Cp));[/asy]
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Let the endpoint of the intersection of the fold near $F$ be $G$ . Since trapezoid $BCFE$ is folded, it is congruent to trapezoid $B'C'FE$ . Therefore, $BE=B'E=17$ . Since $\triangle AB'E$ is a right triangle, $AB'=15$ from the pythagorean theorem. From here, we can see that triangles $\triangle AEB \sim \triangle DGB' \sim \triangle C'GF$ by AA similarity. From here, we find $BC$ from a lot of similarities. Let $BC=x$
Since $\triangle ABE' \sim \triangle DGB'$
\[\frac {AE}{AB'} = \frac{DB}{DG}\]
\[\frac {8}{15} = \frac {x-15}{DG}\]
\[DG = \frac {15(x-15)}{8}\]
\[GF = DC-DG-FC\]
\[GF = \frac{-15x+401}{8}\]
Since $\triangle ABE' \sim \triangle C'GF'$
\[\frac {AE}{B'E} = \frac {C'F}{GF}\]
\[\frac {8}{17} = \frac{3}{\frac {-15x+401}{8}}\]
from which we get $x= \frac {70}{3}$
Finally, our answer is $2(\frac {70}{3}) + 2(25)=\frac {290}{3}$ , which is $290+3=\boxed{293}$
| 293
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6,440
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_8
| 1
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How many positive integer divisors of $2004^{2004}$ are divisible by exactly 2004 positive integers?
|
The prime factorization of 2004 is $2^2\cdot 3\cdot 167$ . Thus the prime factorization of $2004^{2004}$ is $2^{4008}\cdot 3^{2004}\cdot 167^{2004}$
We can count the number of divisors of a number by multiplying together one more than each of the exponents of the prime factors in its prime factorization. For example, the number of divisors of $2004=2^2\cdot 3^1\cdot 167^1$ is $(2+1)(1+1)(1+1)=12$
A positive integer divisor of $2004^{2004}$ will be of the form $2^a\cdot 3^b\cdot 167^c$ . Thus we need to find how many $(a,b,c)$ satisfy
We can think of this as partitioning the exponents to $a+1,$ $b+1,$ and $c+1$ . So let's partition the 2's first. There are two 2's so this is equivalent to partitioning two items in three containers. We can do this in ${4 \choose 2} = 6$ ways. We can partition the 3 in three ways and likewise we can partition the 167 in three ways. So we have $6\cdot 3\cdot 3 = \boxed{54}$ as our answer.
| 54
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6,441
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_8
| 2
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How many positive integer divisors of $2004^{2004}$ are divisible by exactly 2004 positive integers?
|
Clearly we need to find a group of numbers that multiply to 2004. We can list them all out since we know that 2004 is only $167 * 2^2 * 3$
167, 2, 2, 3
4, 3, 167
12, 167
4, 501
2, 1002
2, 3, 334
2, 2, 501*
6, 2, 167
3, 668
6, 334
2004*
To begin, the first multiple doesn't work because there are only 3 prime divisors of 2004. We can apply all multiples because the prime factorization of $2004^{2004}$ is $2^{4008} * 3^{2004} * 167^{2004}.$ Every multiple has six ways of distributing numbers to become powers of 167, 3, and 2, except for the ones with a star.
For a single power of 2004, we have three choices (2, 3, and 167) to give a power of 2003 to.
For 2, 2, 501, there are three choices to give a power of 500 to and the rest get a power of 1.
Therefore we have $8 * 6$ normal multiples and $3 *2$ "half" multiples. Sum to get $\boxed{54}$ as our answer.
| 54
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6,442
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_9
| 1
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sequence of positive integers with $a_1=1$ and $a_9+a_{10}=646$ is formed so that the first three terms are in geometric progression , the second, third, and fourth terms are in arithmetic progression , and, in general, for all $n\ge1,$ the terms $a_{2n-1}, a_{2n}, a_{2n+1}$ are in geometric progression, and the terms $a_{2n}, a_{2n+1},$ and $a_{2n+2}$ are in arithmetic progression. Let $a_n$ be the greatest term in this sequence that is less than $1000$ . Find $n+a_n.$
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Let $x = a_2$ ; then solving for the next several terms, we find that $a_3 = x^2,\ a_4 = x(2x-1),\ a_5$ $= (2x-1)^2,\ a_6$ $= (2x-1)(3x-2)$ , and in general, $a_{2n} = f(n-1)f(n)$ $a_{2n+1} = f(n)^2$ , where $f(n) = nx - (n-1)$
From \[a_9 + a_{10} = f(4)^2 + f(4)f(5) = (4x-3)(9x-7) = 646 = 2\cdot 17 \cdot 19\] , we find that by either the quadratic formula or trial-and-error/modular arithmetic that $x=5$ . Thus $f(n) = 4n+1$ , and we need to find the largest $n$ such that either $f(n)^2\, \mathrm{or}\, f(n)f(n-1) < 1000$ . This happens with $f(7)f(8) = 29 \cdot 33 = 957$ , and this is the $2(8) = 16$ th term of the sequence.
The answer is $957 + 16 = \boxed{973}$
| 973
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6,443
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_9
| 2
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sequence of positive integers with $a_1=1$ and $a_9+a_{10}=646$ is formed so that the first three terms are in geometric progression , the second, third, and fourth terms are in arithmetic progression , and, in general, for all $n\ge1,$ the terms $a_{2n-1}, a_{2n}, a_{2n+1}$ are in geometric progression, and the terms $a_{2n}, a_{2n+1},$ and $a_{2n+2}$ are in arithmetic progression. Let $a_n$ be the greatest term in this sequence that is less than $1000$ . Find $n+a_n.$
|
Let $x = a_2$ . It is apparent that the sequence grows relatively fast, so we start trying positive integers to see what $x$ can be. Finding that $x = 5$ works, after bashing out the rest of the terms we find that $a_{16} = 957$ and $a_{17} = 1089$ , hence our answer is $957 + 16 = \boxed{973}$
| 973
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6,444
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_9
| 3
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sequence of positive integers with $a_1=1$ and $a_9+a_{10}=646$ is formed so that the first three terms are in geometric progression , the second, third, and fourth terms are in arithmetic progression , and, in general, for all $n\ge1,$ the terms $a_{2n-1}, a_{2n}, a_{2n+1}$ are in geometric progression, and the terms $a_{2n}, a_{2n+1},$ and $a_{2n+2}$ are in arithmetic progression. Let $a_n$ be the greatest term in this sequence that is less than $1000$ . Find $n+a_n.$
|
We can find the value of $a_{9}$ by its bounds using three conditions:
Rearranging conditions 1 and 2, we get:
\[\frac{646}{3} < a_{9} < \frac{646}{2}\]
trying all the terms from the third condition, it is clear that $a_9 = 289$ is the only solution.
Then we can calculate the next few terms from there since we have $a_{10}$ as well, to find that $a_{16} = 957$ and $a_{17} = 1089$ , thus we have our answer of $957 + 16 = \boxed{973}$
| 973
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6,445
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_10
| 1
|
Let $S$ be the set of integers between $1$ and $2^{40}$ whose binary expansions have exactly two $1$ 's. If a number is chosen at random from $S,$ the probability that it is divisible by $9$ is $p/q,$ where $p$ and $q$ are relatively prime positive integers. Find $p+q.$
|
Any number from 1 to $2^{40}$ can be represented in binary with 40 digits (because $2^{40}$ has 41) with leading zeroes if necessary. Therefore the number of sets where there are exactly two 1’s in this binary representation is just $\binom {40}{2}$ because we’re choosing 2 1s to go in 40 digit slots. This is equal to 780; we have found $q$ , our denominator.
With these particular numbers, when converting back to decimal note that the 38 zeroes all cancel out. Let the first 1 when reading the number in binary from left to right be in position $y$ , I.e. when converting to decimal, it will have value $2^{y}$ . Similarly, let the second 1 have position $x$ (note $x<y$ ); then the decimal value of that 1 will be $2^{x}$ . For example, with the binary string 0001001000 $y$ is 6 and $x$ is 3 (note that it is zero indexed).
It should also be noted that our constraints on $x$ and $y$ are $0 \leq x < y < 40$ and $x, y \in \mathbb{Z}$
The numbers that satisfy the constraints are therefore equal to $2^{x}+2^{y} = 2^{x} \cdot (1+2^{y-x})$ . The conditions say that this is divisible by 9. $2^{x}$ is clearly never divisible by 9, so we can just focus on $1+2^{y-x}$ \[1+2^{y-x} \equiv 0 \pmod 9 \implies 2^{y-x} \equiv -1 \pmod 9 \implies 2^{y-x} \equiv 8 \pmod 9\]
$y-x = 3$ clearly satisfies the congruence, but are there any more? Well, we see that any time we raise -1 (in the second congruence above) to an odd power we get -1 again, so if we raise $2^{3}$ , which we know already works, to an odd power, we will also satisfy the congruence. Thus, $2^{3}, 2^{9}, 2^{15},$ etc. work; or, \[y-x \equiv 3 \pmod 6\]
To count the numbers that satisfy the condition, we need to find the amount of numbers that satisfy $2^{x} \cdot (1+2^{y-x}) \leq 2^{40}$ , and $y-x \equiv 3 \pmod 6 = 6n +3$ for some whole number $n$
Since we know $1+2^{y-x} = 1+2^{6n+3}$ is positive, we can divide both sides of the inequality by it; \[2^{x} \cdot (1+2^{y-x}) \leq 2^{40} \implies 2^{x} \leq \frac{2^{40}}{1+2^{y-x}} \implies 2^{x} \leq \frac{2^{40}}{1+2^{6n+3}}\] \[\implies 2^{x} < \frac{2^{40}}{2^{6n+3}} \implies 2^{x} < 2^{40-(6n+3)}\implies 2^{x} < 2^{37-6n} \implies x < 37 - 6n\]
Finally, the number of possible values of $x$ is $37-6n$ because $x$ can also be 0 and must be an integer.Therefore, as we increase $n$ from zero upwards, we get the total number of possibilities for $x$ is $37+31+25+\ldots+1 = 7 + 36 + 30 + 24 + \ldots + 6 + 0 = 7 + 6 \cdot (6 + 5 + 4\ldots + 1)$
$= 7 + 6 \cdot 3 \cdot 7 = 7 + 18 \cdot 7 = 19 \cdot 7 = 133$
Since 133 and 780 are relatively prime, the answer is $\frac{133}{780}$ and our final answer is $133+780 = \boxed{913}$
| 913
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6,446
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_10
| 2
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Let $S$ be the set of integers between $1$ and $2^{40}$ whose binary expansions have exactly two $1$ 's. If a number is chosen at random from $S,$ the probability that it is divisible by $9$ is $p/q,$ where $p$ and $q$ are relatively prime positive integers. Find $p+q.$
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A positive integer $n$ has exactly two 1s in its binary representation exactly when $n = 2^j + 2^k$ for $j \neq k$ nonnegative integers. Thus, the set $S$ is equal to the set $\{n \in \mathbb{Z} \mid n = 2^j + 2^k \,\mathrm{ and }\, 0 \leq j < k \leq 39\}$ . (The second condition ensures simultaneously that $j \neq k$ and that each such number less than $2^{40}$ is counted exactly once.) This means there are ${40 \choose 2} = 780$ total such numbers.
Now, consider the powers of $2$ mod $9$ $2^{6n} \equiv 1, 2^{6n + 1} \equiv 2, 2^{6n + 2} \equiv 4, 2^{6n + 3} \equiv 8 \equiv -1,$ $2^{6n + 4} \equiv 7 \equiv -2,$ $2^{6n + 5} \equiv 5 \equiv -4 \pmod 9$
It's clear what the pairs $j, k$ can look like. If one is of the form $6n$ (7 choices), the other must be of the form $6n + 3$ (7 choices). If one is of the form $6n + 1$ (7 choices) the other must be of the form $6n + 4$ (6 choices). And if one is of the form $6n + 2$ (7 choices), the other must be of the form $6n + 5$ (6 choices). This means that there are $7\cdot 7 + 7\cdot 6 + 7\cdot 6 = 49 + 42 +42 = 133$ total "good" numbers.
The probability is $\frac{133}{780}$ , and the answer is $133 + 780 = \boxed{913}$
| 913
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6,447
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_10
| 3
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Let $S$ be the set of integers between $1$ and $2^{40}$ whose binary expansions have exactly two $1$ 's. If a number is chosen at random from $S,$ the probability that it is divisible by $9$ is $p/q,$ where $p$ and $q$ are relatively prime positive integers. Find $p+q.$
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As mentioned above, there are 780 possible combinations. Since we are essentially adding two powers of two together, thinking about the properties of this sum organizes our solution. All powers of two are even except for $2^0$ , so we begin with labeling an entire group "where one of the 1s is in the rightmost spot". In an equation, $2^n+1$ is congruent to 0 mod 9. Instantly we think of $2^3=8$ , and trial and error soon gives us $2^9=512$ . Notice a pattern? Trying 2^15 = 32768 also works. You could go on, but basically all powers of two 3 mod 6 are 1 mod 9. (This is proven with the fact that $2^6$ is 1 mod 9) There are seven 3 mod 6 powers of 2 from 1-39, so for our first category there are seven "good" combinations.
Next, we look at powers of two exclusive from 1-39. Expressed in an equation, $2^n+2^m$ is congruent to 0 mod 9. Since nine is a relatively small number, we can treat powers of two as remainders of nine, to find "good" combinations quickly. Making a list of powers of two from 1-6 shows that the remainders when divided by nine are 2, 4, 8, 7, 5, and 1. This repeats itself every 6 powers. With simple counting we see there are seven powers of two that give remainders of 2, 4, and 8 each. Also there are six powers of two two that give remainders of 7, 5, and 1 each. Notice that each trio of powers correspond with another in the opposite trio. WLOG, if you choose two, (which there are seven to select) there are six different sevens to pair. This gives $7*6$ . Repeat for the other two pairs.
In the end there are $7+42+42+42$ "good" combinations, for an answer of $133 + 780 = \boxed{913}$
| 913
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6,448
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_11
| 1
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right circular cone has a base with radius $600$ and height $200\sqrt{7}.$ A fly starts at a point on the surface of the cone whose distance from the vertex of the cone is $125$ , and crawls along the surface of the cone to a point on the exact opposite side of the cone whose distance from the vertex is $375\sqrt{2}.$ Find the least distance that the fly could have crawled.
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The easiest way is to unwrap the cone into a circular sector. Center the sector at the origin with one radius on the positive $x$ -axis and the angle $\theta$ going counterclockwise. The circumference of the base is $C=1200\pi$ . The sector's radius (cone's sweep) is $R=\sqrt{r^2+h^2}=\sqrt{600^2+(200\sqrt{7})^2}=\sqrt{360000+280000}=\sqrt{640000}=800$ . Setting $\theta R=C\implies 800\theta=1200\pi\implies\theta=\frac{3\pi}{2}$
If the starting point $A$ is on the positive $x$ -axis at $(125,0)$ then we can take the end point $B$ on $\theta$ 's bisector at $\frac{3\pi}{4}$ radians along the $y=-x$ line in the second quadrant. Using the distance from the vertex puts $B$ at $(-375,-375)$ . Thus the shortest distance for the fly to travel is along segment $AB$ in the sector, which gives a distance $\sqrt{(-375-125)^2+(-375-0)^2}=125\sqrt{4^2+3^2}=\boxed{625}$
| 625
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6,449
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_12
| 1
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Let $ABCD$ be an isosceles trapezoid , whose dimensions are $AB = 6, BC=5=DA,$ and $CD=4.$ Draw circles of radius 3 centered at $A$ and $B,$ and circles of radius 2 centered at $C$ and $D.$ A circle contained within the trapezoid is tangent to all four of these circles. Its radius is $\frac{-k+m\sqrt{n}}p,$ where $k, m, n,$ and $p$ are positive integers $n$ is not divisible by the square of any prime , and $k$ and $p$ are relatively prime . Find $k+m+n+p.$
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Let the radius of the center circle be $r$ and its center be denoted as $O$
Clearly line $AO$ passes through the point of tangency of circle $A$ and circle $O$ . Let $y$ be the height from the base of the trapezoid to $O$ . From the Pythagorean Theorem \[3^2 + y^2 = (r + 3)^2 \Longrightarrow y = \sqrt {r^2 + 6r}.\]
We use a similar argument with the line $DO$ , and find the height from the top of the trapezoid to $O$ $z$ , to be $z = \sqrt {r^2 + 4r}$
Now $y + z$ is simply the height of the trapezoid. Let $D'$ be the foot of the perpendicular from $D$ to $AB$ ; then $AD' = 3 - 2 = 1$ . By the Pythagorean Theorem, $(AD')^2 + (DD')^2 = (AD)^2 \Longrightarrow DD' = \sqrt{24}$ so we need to solve the equation $\sqrt {r^2 + 4r} + \sqrt {r^2 + 6r} = \sqrt {24}$ . We can solve this by moving one radical to the other side, and squaring the equation twice to end with a quadratic equation
Solving this, we get $r = \frac { - 60 + 48\sqrt {3}}{23}$ , and the answer is $k + m + n + p = 60 + 48 + 3 + 23 = \boxed{134}$
| 134
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6,450
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_13
| 1
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Let $ABCDE$ be a convex pentagon with $AB \parallel CE, BC \parallel AD, AC \parallel DE, \angle ABC=120^\circ, AB=3, BC=5,$ and $DE = 15.$ Given that the ratio between the area of triangle $ABC$ and the area of triangle $EBD$ is $m/n,$ where $m$ and $n$ are relatively prime positive integers, find $m+n.$
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Let the intersection of $\overline{AD}$ and $\overline{CE}$ be $F$ . Since $AB \parallel CE, BC \parallel AD,$ it follows that $ABCF$ is a parallelogram , and so $\triangle ABC \cong \triangle CFA$ . Also, as $AC \parallel DE$ , it follows that $\triangle ABC \sim \triangle EFD$
By the Law of Cosines $AC^2 = 3^2 + 5^2 - 2 \cdot 3 \cdot 5 \cos 120^{\circ} = 49 \Longrightarrow AC = 7$ . Thus the length similarity ratio between $\triangle ABC$ and $\triangle EFD$ is $\frac{AC}{ED} = \frac{7}{15}$
Let $h_{ABC}$ and $h_{BDE}$ be the lengths of the altitudes in $\triangle ABC, \triangle BDE$ to $AC, DE$ respectively. Then, the ratio of the areas $\frac{[ABC]}{[BDE]} = \frac{\frac 12 \cdot h_{ABC} \cdot AC}{\frac 12 \cdot h_{BDE} \cdot DE} = \frac{7}{15} \cdot \frac{h_{ABC}}{h_{BDE}}$
However, $h_{BDE} = h_{ABC} + h_{CAF} + h_{EFD}$ , with all three heights oriented in the same direction. Since $\triangle ABC \cong \triangle CFA$ , it follows that $h_{ABC} = h_{CAF}$ , and from the similarity ratio, $h_{EFD} = \frac{15}{7}h_{ABC}$ . Hence $\frac{h_{ABC}}{h_{BDE}} = \frac{h_{ABC}}{2h_{ABC} + \frac {15}7h_{ABC}} = \frac{7}{29}$ , and the ratio of the areas is $\frac{7}{15} \cdot \frac 7{29} = \frac{49}{435}$ . The answer is $m+n = \boxed{484}$
| 484
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6,451
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_14
| 1
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Consider a string of $n$ $7$ 's, $7777\cdots77,$ into which $+$ signs are inserted to produce an arithmetic expression . For example, $7+77+777+7+7=875$ could be obtained from eight $7$ 's in this way. For how many values of $n$ is it possible to insert $+$ signs so that the resulting expression has value $7000$
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Suppose we require $a$ $7$ s, $b$ $77$ s, and $c$ $777$ s to sum up to $7000$ $a,b,c \ge 0$ ). Then $7a + 77b + 777c = 7000$ , or dividing by $7$ $a + 11b + 111c = 1000$ . Then the question is asking for the number of values of $n = a + 2b + 3c$
Manipulating our equation, we have $a + 2b + 3c = n = 1000 - 9(b + 12c) \Longrightarrow 0 \le 9(b+12c) < 1000$ . Thus the number of potential values of $n$ is the number of multiples of $9$ from $0$ to $1000$ , or $112$
However, we forgot to consider the condition that $a \ge 0$ . For a solution set $(b,c): n=1000-9(b+12c)$ , it is possible that $a = n-2b-3c < 0$ (for example, suppose we counted the solution set $(b,c) = (1,9) \Longrightarrow n = 19$ , but substituting into our original equation we find that $a = -10$ , so it is invalid). In particular, this invalidates the values of $n$ for which their only expressions in terms of $(b,c)$ fall into the inequality $9b + 108c < 1000 < 11b + 111c$
For $1000 - n = 9k \le 9(7 \cdot 12 + 11) = 855$ , we can express $k$ in terms of $(b,c): n \equiv b \pmod{12}, 0 \le b \le 11$ and $c = \frac{n-b}{12} \le 7$ (in other words, we take the greatest possible value of $c$ , and then "fill in" the remainder by incrementing $b$ ). Then $11b + 111c \le 855 + 2b + 3c \le 855 + 2(11) + 3(7) = 898 < 1000$ , so these values work.
Similarily, for $855 \le 9k \le 9(8 \cdot 12 + 10) = 954$ , we can let $(b,c) = (k-8 \cdot 12,8)$ , and the inequality $11b + 111c \le 954 + 2b + 3c \le 954 + 2(10) + 3(8) = 998 < 1000$ . However, for $9k \ge 963 \Longrightarrow n \le 37$ , we can no longer apply this approach.
So we now have to examine the numbers on an individual basis. For $9k = 972$ $(b,c) = (0,9)$ works. For $9k = 963, 981, 990, 999 \Longrightarrow n = 37, 19, 10, 1$ , we find (using that respectively, $b = 11,9,10,11 + 12p$ for integers $p$ ) that their is no way to satisfy the inequality $11b + 111c < 1000$
Thus, the answer is $112 - 4 = \boxed{108}$
| 108
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6,452
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_14
| 2
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Consider a string of $n$ $7$ 's, $7777\cdots77,$ into which $+$ signs are inserted to produce an arithmetic expression . For example, $7+77+777+7+7=875$ could be obtained from eight $7$ 's in this way. For how many values of $n$ is it possible to insert $+$ signs so that the resulting expression has value $7000$
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To simplify, replace all the $7$ ’s with $1$ ’s.
Because the sum is congruent to $n \pmod 9$ and \[1000 \equiv 1 \pmod 9 \implies n \equiv 1 \pmod 9\] Also, $n \leq 1000$ . There are $\big\lfloor \tfrac{1000}{9} \big\rfloor + 1 = 112$ positive integers that satisfy both conditions i.e. $\{1, 10, 19, 28, 37, 46, . . . , 1000\}.$
For $n = 1, 10, 19$ , the greatest sum that is less than or equal to $1000$ is \[6 \cdot 111 + 1 = 677 \implies 112-3 = 109.\]
Thus $n \geq 28$ and let $S = \{28, 37, 46, . . . , 1000\}$
Note that $n=28$ is possible because $9 \cdot 111+1 \cdot 1 = 1000$
When $n = 37$ , the greatest sum that is at most $1000$ is $8 \cdot 111+6\cdot 11+1 \cdot 1 = 955$
All other elements of $S$ are possible because if any element $n$ of $S$ between $46$ and $991$ is possible, then $(n+ 9)$ must be too.
$\textbf{Case 1:}$ Sum has no $11$ 's
It must have at least one $1$ .
If it has exactly one $1$ , there must be nine $111$ ’s and $n = 28$ .
Thus, for $n \geq 46$ , the sum has more than one $1$ , so it must have at least $1000 - 8 \cdot 111 = 112$ number of $1$ ’s.
For $n \leq 1000$ , at least one $111$ .
To show that if $n$ is possible, then $(n + 9)$ is possible, replace a $111$ with $1 + 1 + 1$ , replace eleven $(1 + 1)$ ’s with eleven $11$ ’s, and include nine new $1$ ’s as $+1$ ’s. The sum remains $1000$
$\textbf{Case 2:}$ Sum has at least one $11$
Replace an $11$ with $1 + 1$ , and include nine new $1$ ’s as $+1$ ’s.
Now note that $46$ is possible because $8 \cdot 111 + 10 \cdot 11 + 2 \cdot 1 = 1000$ .
Thus all elements of $S$ except $37$ are possible.
Thus there are $\boxed{108}$ possible values for $n$
| 108
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6,453
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_14
| 3
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Consider a string of $n$ $7$ 's, $7777\cdots77,$ into which $+$ signs are inserted to produce an arithmetic expression . For example, $7+77+777+7+7=875$ could be obtained from eight $7$ 's in this way. For how many values of $n$ is it possible to insert $+$ signs so that the resulting expression has value $7000$
|
It's obvious that we cannot have any number $\ge 7777$ because $7777 > 7000$ so the max number that an occur is $777$
Let's say we have $a$ 777's , $b$ 77's and $c$ 7's
From here we get our required equation as $777a + 77b + 7c = 7000$
Now comes the main problem , one might think that if we find number of $(a,b,c)$ then we're done , but in reality we are over-counting our number of $n$ 's. This is because $n$ is the total number of 7's and from our equation we'll get $n$ as $3a + 2b + c$ (because there are three 7's , two 7's and one 7)
The reason why we're over-counting is because , say $a_1 , b_1 , c_1$ be a solution of our original equation and $a_2 , b_2 , c_2$ be another solution of our original equation , then there can be a possibility that $3a_1 + 2b_1 + c_1 = 3a_2 + 2b_2 + c_2$ where $a_1 \neq a_2 , b_1 \neq b_2 , c_1 \neq c_2$ (example : $2 + 3 + 4 = 1 + 5 + 3$ but $2 \neq 1 , 3 \neq 5 , 4 \neq 3$
We know that $0 \le a \le 9$ $0 \le b \le 90$ $0 \le c \le 1000$ The bound on $a$ is easier to handle with , so lets start putting values on $a$ and calculate $b , c , n$ by making cases
Reduced equation : $111a + 11b + c = 1000$
Case 1 : $a = 9$
We get $11b + c = 1 \implies b = 0 , c = 1$ is our only solution thus only $\boxed{1}$ value of $n$
| 1
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6,454
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https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_15
| 1
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A long thin strip of paper is $1024$ units in length, $1$ unit in width, and is divided into $1024$ unit squares. The paper is folded in half repeatedly. For the first fold, the right end of the paper is folded over to coincide with and lie on top of the left end. The result is a $512$ by $1$ strip of double thickness. Next, the right end of this strip is folded over to coincide with and lie on top of the left end, resulting in a $256$ by $1$ strip of quadruple thickness. This process is repeated $8$ more times. After the last fold, the strip has become a stack of $1024$ unit squares. How many of these squares lie below the square that was originally the $942$ nd square counting from the left?
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Number the squares $0, 1, 2, 3, ... 2^{k} - 1$ . In this case $k = 10$ , but we will consider more generally to find an inductive solution. Call $s_{n, k}$ the number of squares below the $n$ square after the final fold in a strip of length $2^{k}$
Now, consider the strip of length $1024$ . The problem asks for $s_{941, 10}$ . We can derive some useful recurrences for $s_{n, k}$ as follows: Consider the first fold. Each square $s$ is now paired with the square $2^{k} - s - 1$ . Now, imagine that we relabel these pairs with the indices $0, 1, 2, 3... 2^{k - 1} - 1$ - then the $s_{n, k}$ value of the pairs correspond with the $s_{n, k - 1}$ values - specifically, double, and maybe $+ 1$ (if the member of the pair that you're looking for is the top one at the final step).
So, after the first fold on the strip of length $1024$ , the $941$ square is on top of the $82$ square. We can then write
\[s_{941, 10} = 2s_{82, 9} + 1\]
(We add one because $941$ is the odd member of the pair, and it will be on top. This is more easily visually demonstrated than proven.) We can repeat this recurrence, adding one every time we pair an odd to an even (but ignoring the pairing if our current square is the smaller of the two):
\[s_{82, 9} = 2s_{82, 8} = 4s_{82, 7} = 8s_{127 - 82, 6} = 8s_{45, 6}\]
\[s_{45, 6} = 2s_{63 - 45, 5} + 1 = 2s_{18, 5} + 1 = 4s_{31 - 18, 4} + 1 = 4s_{13, 4} + 1\]
\[s_{13, 4} = 2s_{15 - 13, 3} + 1 = 2s_{2, 3}+1\]
We can easily calculate $s_{2, 3} = 4$ from a diagram. Plugging back in,
\begin{align*} s_{13, 4} &= 9 \\ s_{45, 6} &= 37 \\ s_{82, 9} &= 296 \\ s_{941, 10} &= \boxed{593}
| 593
|
6,455
|
https://artofproblemsolving.com/wiki/index.php/2004_AIME_II_Problems/Problem_15
| 2
|
A long thin strip of paper is $1024$ units in length, $1$ unit in width, and is divided into $1024$ unit squares. The paper is folded in half repeatedly. For the first fold, the right end of the paper is folded over to coincide with and lie on top of the left end. The result is a $512$ by $1$ strip of double thickness. Next, the right end of this strip is folded over to coincide with and lie on top of the left end, resulting in a $256$ by $1$ strip of quadruple thickness. This process is repeated $8$ more times. After the last fold, the strip has become a stack of $1024$ unit squares. How many of these squares lie below the square that was originally the $942$ nd square counting from the left?
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We can keep track of the position of the square labeled 942 in each step. We use an $(x,y)$ coordinate system, so originally the 942 square is in the position $(942,1)$ . In general, suppose that we've folded the strip into an array $r=2^k$ squares wide and $c=1024/r=2^{10-k}$ squares tall (so we've made $10-k$ folds). Then if a square occupies the location $(x,y)$ , we find that after the next fold, it will be in the location described by the procedure \[(x,y)\to\begin{cases}(x,y)&\text{if }x\le 2^{k-1}\\ (r+1-x,2c+1-y)&\text{otherwise}.\end{cases}\] Therefore, we can keep track of the square's location in the following table. \[\begin{array}{c|c|c} (x,y)&\text{rows}&\text{columns}\\\hline (942,1)&1024&1\\ (83,2)&512&2\\ (83,2)&256&4\\ (83,2)&128&8\\ (46,15)&64&16\\ (19,18)&32&32\\ (14,47)&16&64\\ (3,82)&8&128\\ (3,82)&4&256\\ (2,431)&2&512\\ (1,594)&1&1024.\\ \end{array}\] Therefore, at the end of the process, the square labeled 942 will be in the position $(1,594)$ , i.e., it will be above $\boxed{593}$ squares.
| 593
|
6,456
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_1
| 1
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Given that
where $k$ and $n$ are positive integers and $n$ is as large as possible, find $k + n.$
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Note that \[{{\left((3!)!\right)!}\over{3!}}= {{(6!)!}\over{6}}={{720!}\over6}={{720\cdot719!}\over6}=120\cdot719!.\] Because $120\cdot719!<720!$ , we can conclude that $n < 720$ . Thus, the maximum value of $n$ is $719$ . The requested value of $k+n$ is therefore $120+719=\boxed{839}$
| 839
|
6,457
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_2
| 1
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One hundred concentric circles with radii $1, 2, 3, \dots, 100$ are drawn in a plane. The interior of the circle of radius $1$ is colored red, and each region bounded by consecutive circles is colored either red or green, with no two adjacent regions the same color. The ratio of the total area of the green regions to the area of the circle of radius $100$ can be expressed as $m/n,$ where $m$ and $n$ are relatively prime positive integers . Find $m + n.$
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To get the green area, we can color all the circles of radius $100$ or below green, then color all those with radius $99$ or below red, then color all those with radius $98$ or below green, and so forth. This amounts to adding the area of the circle of radius $100$ , but subtracting the circle of radius $99$ , then adding the circle of radius $98$ , and so forth.
The total green area is thus given by $100^{2} \pi - 99^{2} \pi + 98^{2} \pi - \ldots - 1^{2} \pi$ , while the total area is given by $100^{2} \pi$ , so the ratio is \[\frac{100^{2}\pi - 99^{2}\pi + 98^{2}\pi - \ldots - 1^{2}\pi}{100^{2}\pi}\]
For any $a$ $a^{2}-(a-1)^{2}=a^{2}-(a^{2}-2a+1)=2a-1$ . We can cancel the factor of pi from the numerator and denominator and simplify the ratio to
\[\frac{(2\cdot100 - 1)+(2\cdot98 - 1) + \ldots + (2\cdot 2 - 1)}{100^{2}} = \frac{2\cdot(100 + 98 + \ldots + 2) - 50}{100^2}.\]
Using the formula for the sum of an arithmetic series , we see that this is equal to
\[\frac{2(50)(51)-50}{100^{2}}=\frac{50(101)}{100^{2}}=\frac{101}{200},\]
so the answer is $101 + 200 =\boxed{301}$
| 301
|
6,458
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_2
| 2
|
One hundred concentric circles with radii $1, 2, 3, \dots, 100$ are drawn in a plane. The interior of the circle of radius $1$ is colored red, and each region bounded by consecutive circles is colored either red or green, with no two adjacent regions the same color. The ratio of the total area of the green regions to the area of the circle of radius $100$ can be expressed as $m/n,$ where $m$ and $n$ are relatively prime positive integers . Find $m + n.$
|
We want to find $\frac{\sum\limits_{n=1}^{50} (4n-1)\pi}{10000\pi}=\frac{\sum\limits_{n=1}^{50} (4n-1)}{10000}=\frac{(\sum\limits_{n=1}^{50} (4n) )-50}{10000}=\frac{101}{200} \rightarrow 101+200=\boxed{301}$
| 301
|
6,459
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_2
| 3
|
One hundred concentric circles with radii $1, 2, 3, \dots, 100$ are drawn in a plane. The interior of the circle of radius $1$ is colored red, and each region bounded by consecutive circles is colored either red or green, with no two adjacent regions the same color. The ratio of the total area of the green regions to the area of the circle of radius $100$ can be expressed as $m/n,$ where $m$ and $n$ are relatively prime positive integers . Find $m + n.$
|
The sum of the areas of the green regions is
\[\left[(2^2-1^2)+(4^2-3^2)+(6^2-5^2)+\cdots+(100^2-99^2)\right]\pi\] \[=\left[(2+1)+(4+3)+(6+5)+\cdots+(100+99)\right]\pi\] \[={1\over2}\cdot100\cdot101\pi.\]
Thus the desired ratio is \[{1\over2}\cdot{{100\cdot101\pi}\over{100^2\pi}}={101\over200},\] and $m+n=\boxed{301}$
| 301
|
6,460
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_3
| 1
|
Let the set $\mathcal{S} = \{8, 5, 1, 13, 34, 3, 21, 2\}.$ Susan makes a list as follows: for each two-element subset of $\mathcal{S},$ she writes on her list the greater of the set's two elements. Find the sum of the numbers on the list.
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Order the numbers in the set from greatest to least to reduce error: $\{34, 21, 13, 8, 5, 3, 2, 1\}.$ Each element of the set will appear in $7$ two-element subsets , once with each other number.
Therefore the desired sum is $34\cdot7+21\cdot6+13\cdot5+8\cdot4+5\cdot3+3 \cdot2+2\cdot1+1\cdot0=\boxed{484}$
| 484
|
6,461
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_3
| 2
|
Let the set $\mathcal{S} = \{8, 5, 1, 13, 34, 3, 21, 2\}.$ Susan makes a list as follows: for each two-element subset of $\mathcal{S},$ she writes on her list the greater of the set's two elements. Find the sum of the numbers on the list.
|
Thinking of this problem algorithmically, one can "sort" the array to give: \[{1, 2, 3, 5, 8, 13, 21, 34}\]
Now, notice that when we consider different pairs, we are only going to fixate one element and look at the all of the next elements in the array, basically the whole $j = i + 1$ shebang. Then, we see that if we set the sum of the whole array to $x,$ we get out answer to be
\[(x-1) + (x-3) + (x-6) + (x-11) + (x-19) + (x-32) + (x-53) = 7x - 125\]
Finding $7x$ isn't hard, and we see that it is equal to $609$
\[609 - 125 = \boxed{484}\]
| 484
|
6,462
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_5
| 1
|
Consider the set of points that are inside or within one unit of a rectangular parallelepiped (box) that measures $3$ by $4$ by $5$ units. Given that the volume of this set is $\frac{m + n\pi}{p},$ where $m, n,$ and $p$ are positive integers , and $n$ and $p$ are relatively prime , find $m + n + p.$
|
The set can be broken into several parts: the big $3\times 4 \times 5$ parallelepiped, $6$ external parallelepipeds that each share a face with the large parallelepiped and have a height of $1$ , the $1/8$ spheres (one centered at each vertex of the large parallelepiped), and the $1/4$ cylinders connecting each adjacent pair of spheres.
The combined volume of these parts is $60+94+\frac{4}{3}\pi+12\pi = \frac{462+40\pi}{3}$ . Thus, the answer is $m+n+p = 462+40+3 = \boxed{505}$
| 505
|
6,463
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_6
| 1
|
The sum of the areas of all triangles whose vertices are also vertices of a $1$ by $1$ by $1$ cube is $m + \sqrt{n} + \sqrt{p},$ where $m, n,$ and $p$ are integers . Find $m + n + p.$
|
Since there are $8$ vertices of a cube , there are ${8 \choose 3} = 56$ total triangles to consider. They fall into three categories: there are those which are entirely contained within a single face of the cube (whose sides are two edges and one face diagonal ), those which lie in a plane perpendicular to one face of the cube (whose sides are one edge, one face diagonal and one space diagonal of the cube) and those which lie in a plane oblique to the edges of the cube, whose sides are three face diagonals of the cube.
Each face of the cube contains ${4\choose 3} = 4$ triangles of the first type, and there are $6$ faces, so there are $24$ triangles of the first type. Each of these is a right triangle with legs of length $1$ , so each triangle of the first type has area $\frac 12$
Each edge of the cube is a side of exactly $2$ of the triangles of the second type, and there are $12$ edges, so there are $24$ triangles of the second type. Each of these is a right triangle with legs of length $1$ and $\sqrt 2$ , so each triangle of the second type has area $\frac{\sqrt{2}}{2}$
Each vertex of the cube is associated with exactly one triangle of the third type (whose vertices are its three neighbors), and there are $8$ vertices of the cube, so there are $8$ triangles of the third type. Each of the these is an equilateral triangle with sides of length $\sqrt 2$ , so each triangle of the third type has area $\frac{\sqrt 3}2$
Thus the total area of all these triangles is $24 \cdot \frac12 + 24\cdot\frac{\sqrt2}2 + 8\cdot\frac{\sqrt3}2 = 12 + 12\sqrt2 + 4\sqrt3 = 12 + \sqrt{288} + \sqrt{48}$ and the answer is $12 + 288 + 48 = \boxed{348}$
| 348
|
6,464
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_7
| 1
|
Point $B$ is on $\overline{AC}$ with $AB = 9$ and $BC = 21.$ Point $D$ is not on $\overline{AC}$ so that $AD = CD,$ and $AD$ and $BD$ are integers . Let $s$ be the sum of all possible perimeters of $\triangle ACD$ . Find $s.$
|
Denote the height of $\triangle ACD$ as $h$ $x = AD = CD$ , and $y = BD$ . Using the Pythagorean theorem , we find that $h^2 = y^2 - 6^2$ and $h^2 = x^2 - 15^2$ . Thus, $y^2 - 36 = x^2 - 225 \Longrightarrow x^2 - y^2 = 189$ . The LHS is difference of squares , so $(x + y)(x - y) = 189$ . As both $x,\ y$ are integers, $x+y,\ x-y$ must be integral divisors of $189$
The pairs of divisors of $189$ are $(1,189)\ (3,63)\ (7,27)\ (9,21)$ . This yields the four potential sets for $(x,y)$ as $(95,94)\ (33,30)\ (17,10)\ (15,6)$ . The last is not a possibility since it simply degenerates into a line . The sum of the three possible perimeters of $\triangle ACD$ is equal to $3(AC) + 2(x_1 + x_2 + x_3) = 90 + 2(95 + 33 + 17) = \boxed{380}$
| 380
|
6,465
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_8
| 1
|
In an increasing sequence of four positive integers, the first three terms form an arithmetic progression , the last three terms form a geometric progression , and the first and fourth terms differ by $30$ . Find the sum of the four terms.
|
Denote the first term as $a$ , and the common difference between the first three terms as $d$ . The four numbers thus are in the form $a,\ a+d,\ a+2d,\ \frac{(a + 2d)^2}{a + d}$
Since the first and fourth terms differ by $30$ , we have that $\frac{(a + 2d)^2}{a + d} - a = 30$ . Multiplying out by the denominator, \[(a^2 + 4ad + 4d^2) - a(a + d) = 30(a + d).\] This simplifies to $3ad + 4d^2 = 30a + 30d$ , which upon rearranging yields $2d(2d - 15) = 3a(10 - d)$
Both $a$ and $d$ are positive integers, so $2d - 15$ and $10 - d$ must have the same sign. Try if they are both positive (notice if they are both negative , then $d > 10$ and $d < \frac{15}{2}$ , which is a contradiction). Then, $d = 8, 9$ . Directly substituting and testing shows that $d \neq 8$ , but that if $d = 9$ then $a = 18$ . Alternatively, note that $3|2d$ or $3|2d-15$ implies that $3|d$ , so only $9$ may work. Hence, the four terms are $18,\ 27,\ 36,\ 48$ , which indeed fits the given conditions. Their sum is $\boxed{129}$
| 129
|
6,466
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_8
| 2
|
In an increasing sequence of four positive integers, the first three terms form an arithmetic progression , the last three terms form a geometric progression , and the first and fourth terms differ by $30$ . Find the sum of the four terms.
|
The sequence is of the form $a-d,$ $a,$ $a+d,$ $\frac{(a+d)^2}{a}$ . Since the first and last terms differ by 30, we have \[\frac{(a+d)^2}{a}-a+d=30\] \[d^2+3ad=30a\] \[d^2+3ad-30a=0\] \[d=\frac{-3a + \sqrt{9a^2+120a}}{2}.\] Let $9a^2+120a=x^2$ , where $x$ is an integer. This yields the following: \[9a^2+120a-x^2=0\] \[a=\frac{-120 + \sqrt{14400+36x^2}}{18}\] \[a=\frac{-20 + \sqrt{400+x^2}}{3}.\] We then set $400+x^2=y^2$ , where $y$ is an integer. Factoring using difference of squares, we have \[400=2^4 \cdot 5^2=(y+x)(y-x).\] Then, noticing that $y+x > y-x$ , we set up several systems of equations involving the factors of $400$ . The second system we set up in this manner, \[y+x=2^3 \cdot 5^2\] \[y-x=2,\] yields the solution $y=101, x=99$ . Plugging back in, we get that $a=27 \implies d=9$ , so the sequence is $18,$ $27,$ $36,$ $48,$ and the answer is $\boxed{129}.$
| 129
|
6,467
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_8
| 3
|
In an increasing sequence of four positive integers, the first three terms form an arithmetic progression , the last three terms form a geometric progression , and the first and fourth terms differ by $30$ . Find the sum of the four terms.
|
We represent the values as $a-d$ $a$ $a+d$ , and $\frac{(a+d)^2}{a}$ Take the difference between the first and last values \[\frac{(a+d)^2}{a}-a+d=30\] Manipulating the values by expanding and then long division we see \[\frac{a^2+2ad+d^2}{a}-a+d=30\] \[\frac{(a+2d)a+d^2}{a}-a+d=30\] \[a+2d+\frac{d^2}{a}-a+d=30\] Combining like terms we get \[3d+\frac{d^2}{a}=30\] And looking at the values we know that since the second term must be positive (since both a and d are positive), $d$ must be a maximum of 9, which offers 9 possible values (since $d$ must be a positive integer.) We can resort to guesswork by this time, and thus receive the result of of $d=9$ at which $a=27$ . The solution is thus $\boxed{129}.$
| 129
|
6,468
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_9
| 1
|
An integer between $1000$ and $9999$ , inclusive, is called balanced if the sum of its two leftmost digits equals the sum of its two rightmost digits. How many balanced integers are there?
|
If the common sum of the first two and last two digits is $n$ , such that $1 \leq n \leq 9$ , there are $n$ choices for the first two digits and $n + 1$ choices for the second two digits (since zero may not be the first digit). This gives $\sum_{n = 1}^9 n(n + 1) = 330$ balanced numbers. If the common sum of the first two and last two digits is $n$ , such that $10 \leq n \leq 18$ , there are $19 - n$ choices for both pairs. This gives $\sum_{n = 10}^{18} (19 - n)^2 = \sum_{n = 1}^9 n^2 = 285$ balanced numbers. Thus, there are in total $330 + 285 = \boxed{615}$ balanced numbers.
| 615
|
6,469
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_9
| 2
|
An integer between $1000$ and $9999$ , inclusive, is called balanced if the sum of its two leftmost digits equals the sum of its two rightmost digits. How many balanced integers are there?
|
Call the number $\overline{abcd}$ . Then $a+b=c+d$ . Set $a+b=x$
Clearly, $0\le x \le18$
If $x=0$ $0000$ is not acceptable.
If $x=1$ : The only case is $1001$ or $1010$ . 2 choices.
If $x=2$ : then since $a\neq0$ $a=1=b$ or $a=2, b=0$ . There are 3 choices for $(c,d)$ $(2,0), (0, 2), (1, 1)$ $2*3=6$ here.
If $x=3$ : Clearly, $a\neq b$ because if so, the sum will be even, not odd. Counting $(a,b)=(3,0)$ , we have $4$ choices. Subtracting that, we have $3$ choices. Since it doesn't matter whether $c=0$ or $d=0$ , we have 4 choices for $(c,d)$ . So $3*4=12$ here.
If $x=4$ : Continue as above. $4$ choices for $(a,b)$ $5$ choices for $(c,d)$ $4*5=20$ here.
If $x=5$ : You get the point. $5*6=30$
If $x=6$ $6*7=42$
If $x=7$ $7*8=56$
If $x=8$ $8*9=72$
If $x=9$ $9*10=90$
Now we need to be careful because if $x=10$ $(c,d)=(0,10)$ is not valid. However, we don't have to worry about $a\neq0$
If $x=10$ $(a,b)=(1,9), (2, 8), ..., (9, 1)$ . Same thing for $(c,d)$ $9*9=81$
If $x=11$ : We start at $(a,b)= (2,9)$ . So $8*8$
Continue this pattern until $x=18: 1*1=1$ . Add everything up: we have $\boxed{615}$
| 615
|
6,470
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_9
| 3
|
An integer between $1000$ and $9999$ , inclusive, is called balanced if the sum of its two leftmost digits equals the sum of its two rightmost digits. How many balanced integers are there?
|
We ignore the requirement that the first digit is non-zero, and do casework on the sum of the sum of the pairs of digits.
If two digits $a$ and $b$ sum to $0$ , we have $1$ possibility: $(a,b) = (0,0)$
If $a+b = 1$ , we have $2$ possibilities: $(a,b) = (0,1)$ and $(a,b) = (1,0)$
$a+b = 2$ $(0,2)$ $(2,0)$ , and $(1,1)$ are the only $3$ possibilities.
We notice a pattern: for all $k \leq 9$ , there are $k+1$ ordered pairs of digits $(a,b)$ such that $a+b = k$ . Then, testing for $10 \leq k \leq 18$ , we notice that there are $(18 - k) + 1$ ordered pairs with a sum of $k$
So the number of ways to pick $4$ digits satisfying the constraint is \[(1)(1) + (2)(2) \dots (10)(10) + (9)(9) + \dots + (1)(1) = \frac{(10)(11)(21)}{6} + \frac{(9)(10)(19)}{6} = 670\]
We have to subtract out the cases where the first digit is $0$ , which is $1+2+\dots+10 = 55$
So our answer is $670-55 = \boxed{615}$
| 615
|
6,471
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_10
| 1
|
Triangle $ABC$ is isosceles with $AC = BC$ and $\angle ACB = 106^\circ.$ Point $M$ is in the interior of the triangle so that $\angle MAC = 7^\circ$ and $\angle MCA = 23^\circ.$ Find the number of degrees in $\angle CMB.$
|
Take point $N$ inside $\triangle ABC$ such that $\angle CBN = 7^\circ$ and $\angle BCN = 23^\circ$
$\angle MCN = 106^\circ - 2\cdot 23^\circ = 60^\circ$ . Also, since $\triangle AMC$ and $\triangle BNC$ are congruent (by ASA), $CM = CN$ . Hence $\triangle CMN$ is an equilateral triangle , so $\angle CNM = 60^\circ$
Then $\angle MNB = 360^\circ - \angle CNM - \angle CNB = 360^\circ - 60^\circ - 150^\circ = 150^\circ$ . We now see that $\triangle MNB$ and $\triangle CNB$ are congruent. Therefore, $CB = MB$ , so $\angle CMB = \angle MCB = \boxed{83}$
| 83
|
6,472
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_10
| 2
|
Triangle $ABC$ is isosceles with $AC = BC$ and $\angle ACB = 106^\circ.$ Point $M$ is in the interior of the triangle so that $\angle MAC = 7^\circ$ and $\angle MCA = 23^\circ.$ Find the number of degrees in $\angle CMB.$
|
From the givens, we have the following angle measures $m\angle AMC = 150^\circ$ $m\angle MCB = 83^\circ$ . If we define $m\angle CMB = \theta$ then we also have $m\angle CBM = 97^\circ - \theta$ . Then apply the Law of Sines to triangles $\triangle AMC$ and $\triangle BMC$ to get
\[\frac{\sin 150^\circ}{\sin 7^\circ} = \frac{AC}{CM} = \frac{BC}{CM} = \frac{\sin \theta}{\sin (97^\circ - \theta)}\]
Clearing denominators , evaluating $\sin 150^\circ = \frac 12$ and applying one of our trigonometric identities to the result gives
\[\frac{1}{2} \cos (7^\circ - \theta )= \sin 7^\circ \sin \theta\]
and multiplying through by 2 and applying the double angle formulas gives
\[\cos 7^\circ\cos\theta + \sin7^\circ\sin\theta = 2 \sin7^\circ \sin\theta\]
and so $\cos 7^\circ \cos \theta = \sin 7^\circ \sin\theta \Longleftrightarrow \tan 7^{\circ} = \cot \theta$ ; since $0^\circ < \theta < 180^\circ$ , we must have $\theta = 83^\circ$ , so the answer is $\boxed{83}$
| 83
|
6,473
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_10
| 3
|
Triangle $ABC$ is isosceles with $AC = BC$ and $\angle ACB = 106^\circ.$ Point $M$ is in the interior of the triangle so that $\angle MAC = 7^\circ$ and $\angle MCA = 23^\circ.$ Find the number of degrees in $\angle CMB.$
|
Without loss of generality , let $AC = BC = 1$ . Then, using the Law of Sines in triangle $AMC$ , we get $\frac {1}{\sin 150} = \frac {MC}{\sin 7}$ , and using the sine addition formula to evaluate $\sin 150 = \sin (90 + 60)$ , we get $MC = 2 \sin 7$
Then, using the Law of Cosines in triangle $MCB$ , we get $MB^2 = 4\sin^2 7 + 1 - 4\sin 7(\cos 83) = 1$ , since $\cos 83 = \sin 7$ . So triangle $MCB$ is isosceles, and $\angle CMB = \boxed{83}$
| 83
|
6,474
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_10
| 4
|
Triangle $ABC$ is isosceles with $AC = BC$ and $\angle ACB = 106^\circ.$ Point $M$ is in the interior of the triangle so that $\angle MAC = 7^\circ$ and $\angle MCA = 23^\circ.$ Find the number of degrees in $\angle CMB.$
|
Note: A diagram would be much appreciated; I cannot make one since I'm bad at asymptote. Also, please make this less cluttered :) ~tauros
First, take point $E$ outside of $\triangle{ABC}$ so that $\triangle{CEB}$ is equilateral. Then, connect $A$ $C$ , and $M$ to $E$ . Also, let $ME$ intersect $AB$ at $F$ $\angle{MCE} = 83^\circ - 60^\circ = 23^\circ$ $CE = AB$ , and (trivially) $CM = CM$ , so $\triangle{MCE} \cong \triangle{MCA}$ by SAS congruence. Also, $\angle{CMA} = \angle{CME} = 150^\circ$ , so $\angle{AME} = 60^\circ$ , and $AM = ME$ ,
making $\triangle{AME}$ also equilateral. (it is isosceles with a $60^\circ$ angle) $\triangle{MAF} \cong \triangle{EAF}$ by SAS ( $MA = AE$ $AF = AF$ , and $m\angle{MAF} = m\angle{EAF} = 30^\circ$ ), and $\triangle{MAB} \cong \triangle{EAB}$ by SAS ( $MA = AE$ $AB = AB$ , and $m\angle{MAB} = m\angle{EAB} = 30^\circ$ ). Thus, $\triangle{BME}$ is isosceles, with $m\angle{BME} = m\angle{BEM} = 60^\circ + 7^\circ = 67^\circ$ . Also, $\angle{EMB} + \angle {CMB} = \angle{CME} = 150^\circ$ , so $\angle{CME} = 150^\circ - 67^\circ = \boxed{83}$
| 83
|
6,475
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_12
| 1
|
In convex quadrilateral $ABCD, \angle A \cong \angle C, AB = CD = 180,$ and $AD \neq BC.$ The perimeter of $ABCD$ is $640$ . Find $\lfloor 1000 \cos A \rfloor.$ (The notation $\lfloor x \rfloor$ means the greatest integer that is less than or equal to $x.$
|
By the Law of Cosines on $\triangle ABD$ at angle $A$ and on $\triangle BCD$ at angle $C$ (note $\angle C = \angle A$ ),
\[180^2 + AD^2 - 360 \cdot AD \cos A = 180^2 + BC^2 - 360 \cdot BC \cos A\] \[(AD^2 - BC^2) = 360(AD - BC) \cos A\] \[(AD - BC)(AD + BC) = 360(AD - BC) \cos A\] \[(AD + BC) = 360 \cos A\] We know that $AD + BC = 640 - 360 = 280$ $\cos A = \dfrac{280}{360} = \dfrac{7}{9} = 0.777 \ldots$
$\lfloor 1000 \cos A \rfloor = \boxed{777}$
| 777
|
6,476
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_12
| 2
|
In convex quadrilateral $ABCD, \angle A \cong \angle C, AB = CD = 180,$ and $AD \neq BC.$ The perimeter of $ABCD$ is $640$ . Find $\lfloor 1000 \cos A \rfloor.$ (The notation $\lfloor x \rfloor$ means the greatest integer that is less than or equal to $x.$
|
Notice that $AB = CD$ , and $BD = DB$ , and $\angle{DAB} \cong \angle{BCD}$ , so we have side-side-angle matching on triangles $ABD$ and $CDB$ . Since the problem does not allow $\triangle{ABD} \cong \triangle{CDB}$ , we know that $\angle{ADB}$ is not a right angle, and there is a unique other triangle with the matching side-side-angle.
Extend $AD$ to $C'$ so that $\triangle{ABC'}$ is isosceles with $AB = C'B$ . Then notice that $\triangle{DC'B}$ has matching side-side-angle, and yet $\triangle{ADB} \not\cong \triangle{C'DB}$ because $\angle{ADB}$ is not right. Therefore $\triangle{C'DB}$ is the unique triangle mentioned above, so $\triangle{CDB}$ is congruent, in some order of vertices, to $\triangle{C'DB}$ . Since $\triangle{CDB} \cong \triangle{C'DB}$ would imply $\triangle{CDB} = \triangle{C'DB}$ , making quadrilateral $ABCD$ degenerate, we must have $\triangle{CDB} \cong \triangle{C'BD}$
Since the perimeter of $ABCD$ is $640$ $AD + BC = 640 - 180 - 180 = 280$ . Hence $280 = AD + BC = AD + DC'$ . Drop the altitude of $\triangle{ABC'}$ from $B$ and call the foot $P$ . Then right triangle trigonometry on $\triangle{APB}$ shows that $\cos{A} = AP/AB = 140/180 = 7/9$ , so $\lfloor 1000 \cos A \rfloor = \boxed{777}$
| 777
|
6,477
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_13
| 1
|
Let $N$ be the number of positive integers that are less than or equal to $2003$ and whose base- $2$ representation has more $1$ 's than $0$ 's. Find the remainder when $N$ is divided by $1000$
|
In base- $2$ representation, all positive numbers have a leftmost digit of $1$ . Thus there are ${n \choose k}$ numbers that have $n+1$ digits in base $2$ notation, with $k+1$ of the digits being $1$ 's.
In order for there to be more $1$ 's than $0$ 's, we must have $k+1 > \frac{n+1}{2} \implies k > \frac{n-1}{2} \implies k \ge \frac{n}{2}$ . Therefore, the number of such numbers corresponds to the sum of all numbers on or to the right of the vertical line of symmetry in Pascal's Triangle , from rows $0$ to $10$ (as $2003 < 2^{11}-1$ ). Since the sum of the elements of the $r$ th row is $2^r$ , it follows that the sum of all elements in rows $0$ through $10$ is $2^0 + 2^1 + \cdots + 2^{10} = 2^{11}-1 = 2047$ . The center elements are in the form ${2i \choose i}$ , so the sum of these elements is $\sum_{i=0}^{5} {2i \choose i} = 1 + 2 +6 + 20 + 70 + 252 = 351$
The sum of the elements on or to the right of the line of symmetry is thus $\frac{2047 + 351}{2} = 1199$ . However, we also counted the $44$ numbers from $2004$ to $2^{11}-1 = 2047$ . Indeed, all of these numbers have at least $6$ $1$ 's in their base- $2$ representation, as all of them are greater than $1984 = 11111000000_2$ , which has $5$ $1$ 's. Therefore, our answer is $1199 - 44 = 1155$ , and the remainder is $\boxed{155}$
| 155
|
6,478
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_13
| 2
|
Let $N$ be the number of positive integers that are less than or equal to $2003$ and whose base- $2$ representation has more $1$ 's than $0$ 's. Find the remainder when $N$ is divided by $1000$
|
We will count the number of it $< 2^{11}=2048$ instead of $2003$ (In other words, the length of the base-2 representation is at most $11$ . If there are even digits, $2n$ , then the leftmost digit is $1$ , the rest, $2n-1$ , has odd number of digits. In order for the base-2 representation to have more $1$ 's, we will need more $1$ in the remaining $2n-1$ than $0$ 's. Using symmetry, this is equal to $\frac{2^9+2^7+..+2^1}{2}$ Using similar argument where there are odd amount of digits. The remaining even amount of digit must contains the number of $1$ 's at least as the number of $0$ 's. So it's equal to $\frac{\binom{10}{5}+2^{10}+\binom{8}{4}+2^8+\binom{6}{3}+2^6+...+\binom{0}{0}+2^0}{2}$ Summing both cases, we have $\frac{2^0+2^1+..+2^{10}+\binom{0}{0}+..+\binom{10}{5}}{2} = 199$ . There are $44$ numbers between $2004$ and $2047$ inclusive that satisfy it. So the answer is $199-44=\boxed{155}$
| 155
|
6,479
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_15
| 1
|
In $\triangle ABC, AB = 360, BC = 507,$ and $CA = 780.$ Let $M$ be the midpoint of $\overline{CA},$ and let $D$ be the point on $\overline{CA}$ such that $\overline{BD}$ bisects angle $ABC.$ Let $F$ be the point on $\overline{BC}$ such that $\overline{DF} \perp \overline{BD}.$ Suppose that $\overline{DF}$ meets $\overline{BM}$ at $E.$ The ratio $DE: EF$ can be written in the form $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
By the Angle Bisector Theorem, we know that $[CBD]=\frac{169}{289}[ABC]$ . Therefore, by finding the area of triangle $CBD$ , we see that \[\frac{507\cdot BD}{2}\sin\frac{B}{2}=\frac{169}{289}[ABC].\] Solving for $BD$ yields \[BD=\frac{2[ABC]}{3\cdot289\sin\frac{B}{2}}.\] Furthermore, $\cos\frac{B}{2}=\frac{BD}{BF}$ , so \[BF=\frac{BD}{\cos\frac{B}{2}}=\frac{2[ABC]}{3\cdot289\sin\frac{B}{2}\cos\frac{B}{2}}.\] Now by the identity $2\sin\frac{B}{2}\cos\frac{B}{2}=\sin B$ , we get \[BF=\frac{4[ABC]}{3\cdot289\sin B}.\] But then $[ABC]=\frac{360\cdot 507}{2}\sin B$ , so $BF=\frac{240}{289}\cdot 507$ . Thus $BF:FC=240:49$
Now by the Angle Bisector Theorem, $CD=\frac{169}{289}\cdot 780$ , and we know that $MC=\frac{1}{2}\cdot 780$ so $DM:MC=\frac{169}{289}-\frac{1}{2}:\frac{1}{2}=49:289$
We can now use mass points on triangle CBD. Assign a mass of $240\cdot 49$ to point $C$ . Then $D$ must have mass $240\cdot 289$ and $B$ must have mass $49\cdot 49$ . This gives $F$ a mass of $240\cdot 49+49\cdot 49=289\cdot 49$ . Therefore, $DE:EF=\frac{289\cdot 49}{240\cdot 289}=\frac{49}{240}$ , giving us an answer of $\boxed{289}.$
| 289
|
6,480
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_15
| 2
|
In $\triangle ABC, AB = 360, BC = 507,$ and $CA = 780.$ Let $M$ be the midpoint of $\overline{CA},$ and let $D$ be the point on $\overline{CA}$ such that $\overline{BD}$ bisects angle $ABC.$ Let $F$ be the point on $\overline{BC}$ such that $\overline{DF} \perp \overline{BD}.$ Suppose that $\overline{DF}$ meets $\overline{BM}$ at $E.$ The ratio $DE: EF$ can be written in the form $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
Let $\angle{DBM}=\theta$ and $\angle{DBC}=\alpha$ . Then because $BM$ is a median we have $360\sin{(\alpha+\theta)}=507\sin{(\alpha-\theta)}$ . Now we know \[\sin{(\alpha+\theta)}=\sin{\alpha}\cos{\theta}+\sin{\theta}\cos{\alpha}=\dfrac{DF\cdot BD}{BF\cdot BE}+\dfrac{DE\cdot BD}{BE\cdot BF}=\dfrac{BD(DF+DE)}{BF\cdot BE}\] Expressing the area of $\triangle{BEF}$ in two ways we have \[\dfrac{1}{2}BE\cdot BF\sin{(\alpha-\theta)}=\dfrac{1}{2}EF\cdot BD\] so \[\sin{(\alpha-\theta)}=\dfrac{EF\cdot BD}{BF\cdot BE}\] Plugging this in we have \[\dfrac{360\cdot BD(DF+DE)}{BF\cdot BE}=\dfrac{507\cdot BD\cdot EF}{BF\cdot BE}\] so $\dfrac{DF+DE}{EF}=\dfrac{507}{360}$ . But $DF=DE+EF$ , so this simplifies to $1+\dfrac{2DE}{EF}=\dfrac{507}{360}=\dfrac{169}{120}$ , and thus $\dfrac{DE}{EF}=\dfrac{49}{240}$ , and $m+n=\boxed{289}$
| 289
|
6,481
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_15
| 3
|
In $\triangle ABC, AB = 360, BC = 507,$ and $CA = 780.$ Let $M$ be the midpoint of $\overline{CA},$ and let $D$ be the point on $\overline{CA}$ such that $\overline{BD}$ bisects angle $ABC.$ Let $F$ be the point on $\overline{BC}$ such that $\overline{DF} \perp \overline{BD}.$ Suppose that $\overline{DF}$ meets $\overline{BM}$ at $E.$ The ratio $DE: EF$ can be written in the form $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
Firstly, angle bisector theorem yields $\frac{CD}{AD} = \frac{507}{360} = \frac{169}{120}$ . We're given that $AM=MC$ . Therefore, the cross ratio
\[(A,C;M,D) = \frac{AM(CD)}{AD(MC)} = \frac{169}{120}\]
We need a fourth point for this cross ratio to be useful, so reflect point $F$ over angle bisector $BD$ to a point $F'$ . Then $\triangle BFF'$ is isosceles and $BD$ is an altitude so $DF = DF'$ . Therefore,
\[(A,C;M,D) = (F,F';D,E) \implies \frac{FD(EF')}{EF(DF')} = \frac{EF'}{EF} = \frac{169}{120}\]
All that's left is to fiddle around with the ratios:
\[\frac{EF'}{EF} = \frac{ED+DF'}{EF} = \frac{EF+2DF}{EF} = 1\ +\ 2\left(\frac{DF}{EF}\right) \implies \frac{DF}{EF} = \frac{49}{240} \implies \boxed{289}\]
| 289
|
6,482
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_I_Problems/Problem_15
| 4
|
In $\triangle ABC, AB = 360, BC = 507,$ and $CA = 780.$ Let $M$ be the midpoint of $\overline{CA},$ and let $D$ be the point on $\overline{CA}$ such that $\overline{BD}$ bisects angle $ABC.$ Let $F$ be the point on $\overline{BC}$ such that $\overline{DF} \perp \overline{BD}.$ Suppose that $\overline{DF}$ meets $\overline{BM}$ at $E.$ The ratio $DE: EF$ can be written in the form $m/n,$ where $m$ and $n$ are relatively prime positive integers. Find $m + n.$
|
Extend $DF$ to intersect with the extension of $AB$ at $G$ . Notice that $\triangle{BDF} \cong \triangle{BDG}$ , so $GD=DF$ . We now use Menelaus on $\triangle{GBF}$ , as $A$ $D$ , and $C$ are collinear; this gives us $\frac{GA}{BA} \cdot \frac{BC}{FC} \cdot \frac{DF}{GD}=1$ . As $GD=DF$ , we have $\frac{GA}{AB}=\frac{FC}{BC}$ , hence $\frac{GA}{120}=\frac{FC}{169}$ . Reflect $G$ over $A$ to $G'$ . Note that $\frac{G'A}{BA}=\frac{FC}{BC}$ , and reflexivity, hence $\triangle{ABC} \sim \triangle{BG'F}$ . It's easily concluded from this that $G'F \parallel AC$ , hence $G'F \parallel AD$ . As $GD=DF$ , we have $AD$ is a midsegment of $\triangle{GG'F}$ , thus $G'F = 2AD$ . We now focus on the ratio $\frac{BF}{BC}$ . From similarity, we have $\frac{BF}{BC}=\frac{G'F}{AC}=\frac{2AD}{AC}$ . By the angle bisector theorem, we have $AD:DC=120:169$ , hence $AD:AC=120:289$ , so $\frac{BF}{BC}=\frac{240}{289}$ . We now work out the ratio $\frac{DM}{MC}$ $\frac{DM}{MC}=\frac{CD-MC}{MC}=\frac{CD}{MC}-1=\frac{2CD}{AC}-1=\frac{338}{289}-1=\frac{49}{289}$ . We now use mass points on $\triangle{BDC}$ . We let the mass of $C$ be $240\cdot 49$ , so the mass of $B$ is $49 \cdot 49$ and the mass of $D$ is $289\cdot 240$ . Hence, the mass of $F$ is $289\cdot 49$ , so the ratio $\frac{DE}{EF}=\frac{49}{240}$ . Extracting gives $49+240=\boxed{289}.$
| 289
|
6,483
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_1
| 1
|
The product $N$ of three positive integers is $6$ times their sum , and one of the integers is the sum of the other two. Find the sum of all possible values of $N$
|
Let the three integers be $a, b, c$ $N = abc = 6(a + b + c)$ and $c = a + b$ . Then $N = ab(a + b) = 6(a + b + a + b) = 12(a + b)$ . Since $a$ and $b$ are positive, $ab = 12$ so $\{a, b\}$ is one of $\{1, 12\}, \{2, 6\}, \{3, 4\}$ so $a + b$ is one of $13, 8, 7$ so the sum of all possible values of $N$ is $12 \cdot (13 + 8 + 7) = 12(28) = \boxed{336}$
| 336
|
6,484
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_2
| 1
|
Let $N$ be the greatest integer multiple of 8, no two of whose digits are the same. What is the remainder when $N$ is divided by 1000?
|
We want a number with no digits repeating, so we can only use the digits $0-9$ once in constructing our number. To make the greatest number, we want the greatest digit to occupy the leftmost side and the least digit to occupy the rightmost side. Therefore, the last three digits of the greatest number should be an arrangement of the digits $0,1,2$ . Since the number has to be divisible by 8, the integer formed by the arrangement of $0,1,2$ is also divisible by 8. The only arrangement that works is $120$
Therefore, the remainder when the number is divided by $1000$ is $\boxed{120}$
| 120
|
6,485
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_3
| 1
|
Define a $\text{good~word}$ as a sequence of letters that consists only of the letters $A$ $B$ , and $C$ - some of these letters may not appear in the sequence - and in which $A$ is never immediately followed by $B$ $B$ is never immediately followed by $C$ , and $C$ is never immediately followed by $A$ . How many seven-letter good words are there?
|
There are three letters to make the first letter in the sequence. However, after the first letter (whatever it is), only two letters can follow it, since one of the letters is restricted. Therefore, the number of seven-letter good words is $3*2^6=192$
Therefore, there are $\boxed{192}$ seven-letter good words.
| 192
|
6,486
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_3
| 2
|
Define a $\text{good~word}$ as a sequence of letters that consists only of the letters $A$ $B$ , and $C$ - some of these letters may not appear in the sequence - and in which $A$ is never immediately followed by $B$ $B$ is never immediately followed by $C$ , and $C$ is never immediately followed by $A$ . How many seven-letter good words are there?
|
There are three choices for the first letter and two choices for each subsequent letter, so there are $3\cdot2^{n-1}\ n$ -letter good words. Substitute $n=7$ to find there are $3\cdot2^6=\boxed{192}$ seven-letter good words. ~ aopsav (Credit to AoPS Alcumus)
| 192
|
6,487
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_4
| 1
|
In a regular tetrahedron the centers of the four faces are the vertices of a smaller tetrahedron. The ratio of the volume of the smaller tetrahedron to that of the larger is $m/n$ , where $m$ and $n$ are relatively prime positive integers. Find $m+n$
|
Embed the tetrahedron in 4-space to make calculations easier.
Its vertices are $(1,0,0,0)$ $(0,1,0,0)$ $(0,0,1,0)$ $(0,0,0,1)$
To get the center of any face, we take the average of the three coordinates of that face. The vertices of the center of the faces are: $(\frac{1}{3}, \frac{1}{3}, \frac{1}{3}, 0)$ $(\frac{1}{3}, \frac{1}{3},0, \frac{1}{3})$ $(\frac{1}{3},0, \frac{1}{3}, \frac{1}{3})$ $(0,\frac{1}{3}, \frac{1}{3}, \frac{1}{3})$
The side length of the large tetrahedron is $\sqrt{2}$ by the distance formula.
The side length of the smaller tetrahedron is $\frac{\sqrt{2}}{3}$ by the distance formula.
Their ratio is $1:3$ , so the ratio of their volumes is $\left(\frac{1}{3}\right)^3 = \frac{1}{27}$
$m+n = 1 + 27 = \boxed{28}$
| 28
|
6,488
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_5
| 1
|
A cylindrical log has diameter $12$ inches. A wedge is cut from the log by making two planar cuts that go entirely through the log. The first is perpendicular to the axis of the cylinder, and the plane of the second cut forms a $45^\circ$ angle with the plane of the first cut. The intersection of these two planes has exactly one point in common with the log. The number of cubic inches in the wedge can be expressed as $n\pi$ , where n is a positive integer. Find $n$
|
The volume of the wedge is half the volume of a cylinder with height $12$ and radius $6$ . (Imagine taking another identical wedge and sticking it to the existing one). Thus, $V=\dfrac{6^2\cdot 12\pi}{2}=216\pi$ , so $n=\boxed{216}$
| 216
|
6,489
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_6
| 1
|
In triangle $ABC,$ $AB = 13,$ $BC = 14,$ $AC = 15,$ and point $G$ is the intersection of the medians. Points $A',$ $B',$ and $C',$ are the images of $A,$ $B,$ and $C,$ respectively, after a $180^\circ$ rotation about $G.$ What is the area of the union of the two regions enclosed by the triangles $ABC$ and $A'B'C'?$
|
Since a $13-14-15$ triangle is a $5-12-13$ triangle and a $9-12-15$ triangle "glued" together on the $12$ side, $[ABC]=\frac{1}{2}\cdot12\cdot14=84$
There are six points of intersection between $\Delta ABC$ and $\Delta A'B'C'$ . Connect each of these points to $G$
[asy] size(8cm); pair A,B,C,G,D,E,F,A_1,A_2,B_1,B_2,C_1,C_2; B=(0,0); A=(5,12); C=(14,0); E=(12.6667,8); D=(7.6667,-4); F=(-1.3333,8); G=(6.3333,4); B_1=(4.6667,0); B_2=(1.6667,4); A_1=(3.3333,8); A_2=(8,8); C_1=(11,4); C_2=(9.3333,0); dot(A); dot(B); dot(C); dot(G); dot(D); dot(E); dot(F); dot(A_1); dot(B_1); dot(C_1); dot(A_2); dot(B_2); dot(C_2); draw(B--A--C--cycle); draw(E--D--F--cycle); draw(B_1--A_2); draw(A_1--C_2); draw(C_1--B_2); label("$B$",B,WSW); label("$A$",A,N); label("$C$",C,ESE); label("$G$",G,S); label("$B'$",E,ENE); label("$A'$",D,S); label("$C'$",F,WNW); [/asy]
There are $12$ smaller congruent triangles which make up the desired area. Also, $\Delta ABC$ is made up of $9$ of such triangles.
Therefore, $\left[\Delta ABC \bigcup \Delta A'B'C'\right] = \frac{12}{9}[\Delta ABC]= \frac{4}{3}\cdot84=\boxed{112}$
| 112
|
6,490
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_6
| 2
|
In triangle $ABC,$ $AB = 13,$ $BC = 14,$ $AC = 15,$ and point $G$ is the intersection of the medians. Points $A',$ $B',$ and $C',$ are the images of $A,$ $B,$ and $C,$ respectively, after a $180^\circ$ rotation about $G.$ What is the area of the union of the two regions enclosed by the triangles $ABC$ and $A'B'C'?$
|
First, find the area of $\Delta ABC$ either like the first solution or by using Heron’s Formula. Then, draw the medians from $G$ to each of $A, B, C, A’, B’,$ and $C’$ . Since the medians of a triangle divide the triangle into 6 triangles with equal area, we can find that each of the 6 outer triangles have equal area. (Proof: Since I’m too lazy to draw out a diagram, I’ll just have you borrow the one above. Draw medians $GA$ and $GB’$ , and let’s call the points that $GA$ intersects $C’B’$ $H$ ” and the point $GB’$ intersects $AC$ $I$ ”. From the previous property and the fact that both $\Delta ABC$ and $\Delta A’B’C’$ are congruent, $\Delta GHB’$ has the same area as $\Delta GIA$ . Because of that, both “half” triangles created also have the same area. The same logic can be applied to all other triangles).
Also, since the centroid of a triangle divides each median with the ratio $2:1$ , along with the previous fact, each outer triangle has $1/9$ the area of $\Delta ABC$ and $\Delta A’B’C’$ . Thus, the area of the region required is $\frac{4}{3}$ times the area of $\Delta ABC$ which is $\boxed{112}$
| 112
|
6,491
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_7
| 1
|
Find the area of rhombus $ABCD$ given that the circumradii of triangles $ABD$ and $ACD$ are $12.5$ and $25$ , respectively.
|
The diagonals of the rhombus perpendicularly bisect each other. Call half of diagonal BD $a$ and half of diagonal AC $b$ . The length of the four sides of the rhombus is $\sqrt{a^2+b^2}$
The area of any triangle can be expressed as $\frac{a\cdot b\cdot c}{4R}$ , where $a$ $b$ , and $c$ are the sides and $R$ is the circumradius. Thus, the area of $\triangle ABD$ is $ab=2a(a^2+b^2)/(4\cdot12.5)$ . Also, the area of $\triangle ABC$ is $ab=2b(a^2+b^2)/(4\cdot25)$ . Setting these two expressions equal to each other and simplifying gives $b=2a$ . Substitution yields $a=10$ and $b=20$ , so the area of the rhombus is $20\cdot40/2=\boxed{400}$
| 400
|
6,492
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_7
| 2
|
Find the area of rhombus $ABCD$ given that the circumradii of triangles $ABD$ and $ACD$ are $12.5$ and $25$ , respectively.
|
Let $\theta=\angle BDA$ . Let $AB=BC=CD=x$ . By the extended law of sines, \[\frac{x}{\sin\theta}=25\] Since $AC\perp BD$ $\angle CAD=90-\theta$ , so \[\frac{x}{\sin(90-\theta)=\cos\theta}=50\] Hence $x=25\sin\theta=50\cos\theta$ . Solving $\tan\theta=2$ $\sin\theta=\frac{2}{\sqrt{5}}, \cos\theta=\frac{1}{\sqrt{5}}$ . Thus \[x=25\frac{2}{\sqrt{5}}\implies x^2=500\] The height of the rhombus is $x\sin(2\theta)=2x\sin\theta\cos\theta$ , so we want \[2x^2\sin\theta\cos\theta=\boxed{400}\]
| 400
|
6,493
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_8
| 1
|
Find the eighth term of the sequence $1440,$ $1716,$ $1848,\ldots,$ whose terms are formed by multiplying the corresponding terms of two arithmetic sequences.
|
If you multiply the corresponding terms of two arithmetic sequences, you get the terms of a quadratic function. Thus, we have a quadratic $ax^2+bx+c$ such that $f(1)=1440$ $f(2)=1716$ , and $f(3)=1848$ . Plugging in the values for x gives us a system of three equations:
$a+b+c=1440$
$4a+2b+c=1716$
$9a+3b+c=1848$
Solving gives $a=-72, b=492,$ and $c=1020$ . Thus, the answer is $-72(8)^2+492\cdot8+1020= \boxed{348}.$
| 348
|
6,494
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_8
| 2
|
Find the eighth term of the sequence $1440,$ $1716,$ $1848,\ldots,$ whose terms are formed by multiplying the corresponding terms of two arithmetic sequences.
|
Use the same rationale as in solution 1; instead of using terms $1,2,3$ , we use $-1,0,1$ and solve the $6$ th term.
$a-b+c=1440$
$c=1716$
$a+b+c=1848$
Accordingly we will solve
$a=-72, b=204, c=1716$
$36a+6b+c= \boxed{348}.$
| 348
|
6,495
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_8
| 3
|
Find the eighth term of the sequence $1440,$ $1716,$ $1848,\ldots,$ whose terms are formed by multiplying the corresponding terms of two arithmetic sequences.
|
Setting one of the sequences as $a+nr_1$ and the other as $b+nr_2$ , we can set up the following equalities
$ab = 1440$
$(a+r_1)(b+r_2)=1716$
$(a+2r_1)(b+2r_2)=1848$
We want to find $(a+7r_1)(b+7r_2)$
Foiling out the two above, we have
$ab + ar_2 + br_1 + r_1r_2 = 1716$ and $ab + 2ar_2 + 2br_1 + 4r_1r_2 = 1848$
Plugging in $ab=1440$ and bringing the constant over yields
$ar_2 + br_1 + r_1r_2 = 276$
$ar_2 + br_1 + 2r_1r_2 = 204$
Subtracting the two yields $r_1r_2 = -72$ and plugging that back in yields $ar_2 + br_1 = 348$
Now we find
$(a+7r_1)(b+7r_2) = ab + 7(ar_2 + br_1) + 49r_1r_2 = 1440 + 7(348) + 49(-72) = \boxed{348}$
| 348
|
6,496
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_9
| 2
|
Consider the polynomials $P(x) = x^{6} - x^{5} - x^{3} - x^{2} - x$ and $Q(x) = x^{4} - x^{3} - x^{2} - 1.$ Given that $z_{1},z_{2},z_{3},$ and $z_{4}$ are the roots of $Q(x) = 0,$ find $P(z_{1}) + P(z_{2}) + P(z_{3}) + P(z_{4}).$
|
Let $S_k=z_1^k+z_2^k+z_3^k+z_4^k$ then by Vieta's Formula we have \[S_{-1}=\frac{z_1z_2z_3+z_1z_3z_4+z_1z_2z_4+z_1z_2z_3}{z_1z_2z_3z_4}=0\] \[S_0=4\] \[S_1=1\] \[S_2=3\] By Newton's Sums we have \[a_4S_k+a_3S_{k-1}+a_2S_{k-2}+a_1S_{k-1}+a_0S_{k-4}=0\]
Applying the formula couples of times yields $P(z_1)+P(z_2)+P(z_3)+P(z_4)=S_6-S_5-S_3-S_2-S_1=\boxed{6}$
| 6
|
6,497
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_10
| 1
|
Two positive integers differ by $60$ . The sum of their square roots is the square root of an integer that is not a perfect square. What is the maximum possible sum of the two integers?
|
Call the two integers $b$ and $b+60$ , so we have $\sqrt{b}+\sqrt{b+60}=\sqrt{c}$ . Square both sides to get $2b+60+2\sqrt{b^2+60b}=c$ . Thus, $b^2+60b$ must be a square, so we have $b^2+60b=n^2$ , and $(b+n+30)(b-n+30)=900$ . The sum of these two factors is $2b+60$ , so they must both be even. To maximize $b$ , we want to maximixe $b+n+30$ , so we let it equal $450$ and the other factor $2$ , but solving gives $b=196$ , which is already a perfect square, so we have to keep going. In order to keep both factors even, we let the larger one equal $150$ and the other $6$ , which gives $b=48$ . This checks, so the solution is $48+108=\boxed{156}$
| 156
|
6,498
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_11
| 1
|
Triangle $ABC$ is a right triangle with $AC = 7,$ $BC = 24,$ and right angle at $C.$ Point $M$ is the midpoint of $AB,$ and $D$ is on the same side of line $AB$ as $C$ so that $AD = BD = 15.$ Given that the area of triangle $CDM$ may be expressed as $\frac {m\sqrt {n}}{p},$ where $m,$ $n,$ and $p$ are positive integers, $m$ and $p$ are relatively prime, and $n$ is not divisible by the square of any prime, find $m + n + p.$
|
We use the Pythagorean Theorem on $ABC$ to determine that $AB=25.$
Let $N$ be the orthogonal projection from $C$ to $AB.$ Thus, $[CDM]=\frac{(DM)(MN)} {2}$ $MN=BN-BM$ , and $[ABC]=\frac{24 \cdot 7} {2} =\frac{25 \cdot (CN)} {2}.$
From the third equation, we get $CN=\frac{168} {25}.$
By the Pythagorean Theorem in $\Delta BCN,$ we have
$BN=\sqrt{\left(\frac{24 \cdot 25} {25}\right)^2-\left(\frac{24 \cdot 7} {25}\right)^2}=\frac{24} {25}\sqrt{25^2-7^2}=\frac{576} {25}.$
Thus, $MN=\frac{576} {25}-\frac{25} {2}=\frac{527} {50}.$
In $\Delta ADM$ , we use the Pythagorean Theorem to get $DM=\sqrt{15^2-\left(\frac{25} {2}\right)^2}=\frac{5} {2} \sqrt{11}.$
Thus, $[CDM]=\frac{527 \cdot 5\sqrt{11}} {50 \cdot 2 \cdot 2}= \frac{527\sqrt{11}} {40}.$
Hence, the answer is $527+11+40=\boxed{578}.$
| 578
|
6,499
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_11
| 2
|
Triangle $ABC$ is a right triangle with $AC = 7,$ $BC = 24,$ and right angle at $C.$ Point $M$ is the midpoint of $AB,$ and $D$ is on the same side of line $AB$ as $C$ so that $AD = BD = 15.$ Given that the area of triangle $CDM$ may be expressed as $\frac {m\sqrt {n}}{p},$ where $m,$ $n,$ and $p$ are positive integers, $m$ and $p$ are relatively prime, and $n$ is not divisible by the square of any prime, find $m + n + p.$
|
Let $E$ be the intersection of lines $BC$ and $DM$ . Since triangles $\Delta CME$ and $\Delta CMD$ share a side and height, the area of $\Delta CDM$ is equal to $\frac{DM}{EM}$ times the area of $\Delta CME$ .
By AA similarity, $\Delta EMB$ is similar to $\Delta ACB$ $\frac{EM}{AC}=\frac{MB}{CB}$ . Solving yields $EM=\frac{175}{48}$ . Using the same method but for $EB$ yields $EB=\frac{625}{48}$ . As in previous solutions, by the Pythagorean Theorem $DM=\frac{5\sqrt{11}}{2}$ . So, $\frac{DM}{EM}=\frac{24\sqrt{11}}{35}$ .
Now, since we know both $CB$ and $EB$ , we can find that $CE=\frac{527}{48}$ . The height of $\Delta CME$ is the length from point $M$ to $CB$ . Since $M$ is the midpoint of $AB$ , the height is just $\frac{1}{2}\cdot7=\frac{7}{2}$ . Using this, we can find that the area of $\Delta CMD$ is $\frac{1}{2}\cdot\frac{7}{2}\cdot\frac{527}{48}\cdot\frac{24\sqrt{11}}{35}=\frac{527\sqrt{11}}{40}$ , giving our answer of $\boxed{578}$
| 578
|
6,500
|
https://artofproblemsolving.com/wiki/index.php/2003_AIME_II_Problems/Problem_12
| 1
|
The members of a distinguished committee were choosing a president, and each member gave one vote to one of the 27 candidates. For each candidate, the exact percentage of votes the candidate got was smaller by at least 1 than the number of votes for that candidate. What was the smallest possible number of members of the committee?
|
Let $v_i$ be the number of votes candidate $i$ received, and let $s=v_1+\cdots+v_{27}$ be the total number of votes cast. Our goal is to determine the smallest possible $s$
Candidate $i$ got $\frac{v_i}s$ of the votes, hence the percentage of votes they received is $\frac{100v_i}s$ . The condition in the problem statement says that $\forall i: \frac{100v_i}s + 1 \leq v_i$ . ( $\forall$ means "for all", so this means "For all $i$ $\frac{100v_i}s + 1 \leq v_i$ is true")
Obviously, if some $v_i$ would be $0$ or $1$ , the condition would be false. Thus $\forall i: v_i\geq 2$ . We can then rewrite the above inequality as $\forall i: s\geq\frac{100v_i}{v_i-1}$
If for some $i$ we have $v_i=2$ , then from the inequality we just derived we would have $s\geq 200$
If for some $i$ we have $v_i=3$ , then $s\geq 150$
And if for some $i$ we have $v_i=4$ , then $s\geq \frac{400}3 = 133\frac13$ , and hence $s\geq 134$
Is it possible to have $s<134$ ? We just proved that to have such $s$ , all $v_i$ have to be at least $5$ . But then $s=v_1+\cdots+v_{27}\geq 27\cdot 5 = 135$ , which is a contradiction. Hence the smallest possible $s$ is at least $134$
Now consider a situation where $26$ candidates got $5$ votes each, and one candidate got $4$ votes. In this situation, the total number of votes is exactly $134$ , and for each candidate the above inequality is satisfied. Hence the minimum number of committee members is $s=\boxed{134}$
| 134
|
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