Question 9 In the proof mentioned in the question above, which of the following rules was used? O The Product Rule O The Chain Rule O The Quotient Rule O The Golden Rule Question 10 If a function is written explicitly as z=f(x,y), then a normal vector to the surface can be found by which of the following? (there may be multiple correct answers; select all possible correct answers) Note fx = partial derivative of f with respect to x fy = partial derivative of f with respect to y O O " O O Question 11 Evaluate the gradient of the function f (x, y, z) = xe - ye + ze at the point (0, 0, 0). O O O O O