f(x, y, z) = x^2yz - xyz^3, P(2, -1, 1), u = (0, 4/5, -3/5)
(a) Find the gradient of f.
∇f(x, y, z) = (∂f/∂x, ∂f/∂y, ∂f/∂z)
(b) Evaluate the gradient at the point P.
∇f(2, -1, 1) = (∂f/∂x, ∂f/∂y, ∂f/∂z) at (2, -1, 1)
(c) Find the rate of change of f at P in the direction of the vector u.
Duf(2, -1, 1) = ∇f(2, -1, 1) · u