Let f(x, y) = xy3 − x 2 + 2y − 1. (a) Find the gradient vector
of f(x, y) at the point (2, 1).
(b) Find the directional derivative of f(x, y) at the point (2,
1) in the direction of ~u = 1 √ 10 (3i + j).
(c) Find the directional derivative of f(x, y) at point (2, 1)
in the direction of ~v = 3i + 2j.