15. Assuming that the equation xy + 3z = cos(z2) defines z implicitly as a function of x and y, find oz 2 2 - y2 A. 3-sin(z2) B. - y2 2 2 -y2 3+ sin(z2) C. 3+2z sin(z2) D. 3+2z sin(22 ) E. 3-2z sin(z2) 16. If f(x, y) = xy2, then Vf (2, 3) = A. 127 + 97 B. 182 + 183 C. 97 + 12, D. 21 E. V2. 17. Find the directional derivative of f (x, y) = 5 - 4x2 - 3y at (x, y) towards the origin A. -8x - 3 B. -8x2-3y C. - -8x-3 D. 8x2 + 3y E. 8x-+3y V64x2+9 Va2ty2