fixed gd_bivariate bug

backend/src/optimization_logic.py

@@ -86,8 +86,10 @@ def gd_bivariate(
             mx = momentum * (x_values[-1] - x_values[-2])
             my = momentum * (y_values[-1] - y_values[-2])
 
-        x = x - learning_rate * fx(x, y) + mx
-        y = y - learning_rate * fy(x, y) + my
+        x_new = x - learning_rate * fx(x, y) + mx
+        y_new = y - learning_rate * fy(x, y) + my
+        x = x_new
+        y = y_new
         x_values.append(x)
         y_values.append(y)
         z_values.append(f(x, y))