In Problems 1-3, find the gradient vector of the given function at the given point p. then find the equation of the tangent plane at p.
1. ((x, y) = x2y - xy2, p = (-2, 3)
2. ((x, y) = x3y + 3xy2, p = (2, -2)
3. ((x, y) = cos (x sin (y + sin 2 (y, p = (-, 1/2)