For each function f given below, find all points (a, b) at which fx(x, y) and fy(x, y) are both zero. Determine whether f(a, b) is a local maximum, a local minimum, or neither.
(To do this, consider the sign of f(a + h, b + k) -f(a, b).)
(a) f(x, y) = x2+ 2xy + 2y2-6x + 8y (b) f(x, y) = xy -2x -3y -4