at In problems 10 and 1 1, (a) find the gradient of f; (b) Evaluate the gradient at the given point P ; (c) Find the rate of change of fat P in the direction of the vector u, i.e. the directional derivative. Problem 10. (11.6/8) f(x, y) = Y x , P(1, 2), u = (21 + V/5j) Problem 11. (11.6/10) f(x, y,z) = x+ yz, P(1,3,1), u=(7,7,7)