use Definition 9.5.1 to find Du f(x, y) given that u makes the indicated angle with the positive x- axis. f(x, y) = 3x y2; 0 = 45 - THEOREM 9.5.1 Computing a Directional Derivative If z = f(x, y) is a differentiable function of x and y and u = cos 0 i + sin 0 j, then Du f(x, y) = f(x, y) u. (5)