18. [-/1 Points] DETAILS SCALC9 14.6.507.XP. Find the directional derivative of the function at the given point in the direction of the vector v. f ( x, y, z) = vxyz, (4, 2, 8), V = (-1, -2, 2) DJf ( 4, 2, 8 ) = Need Help? Read It 19. [-/2 Points] DETAILS SCALC9 14.6.510.XP. Find the maximum rate of change of f at the given point and the direction in which it occurs. f ( x , y, z ) = V x2 + 12+ z2, (2, 6, -9) maximum rate of change direction vector Need Help? Read It Watch It 20. [-/2 Points] DETAILS SCALC9 14.6.509.XP. Find the maximum rate of change of fat the given point and the direction in which it occurs. f ( x, y, Z) = 4x + by (3, 4, -1) Z maximum rate of change direction vector