sin(xy+x) (a) Let f(x, y) = x+y2 if x + y 0, 0 if x + y = 0. (i) Determine whether or not f(x, y) is continuous at (0,0). a (ii) Find the partial derivatives of (0,0) and of (0,0), and determine whether af or not f(x, y) is differentiable at (0,0). (b) Let f(x, y) = 3+ y. Find the partial derivatives of (0,0) and (0,0), and show that f(x, y) is not differentiable at (0,0). .4 (c) Let f(x, y) = 4+ y. Find the partial derivatives (0,0) and (0,0), and show that f(x, y) is differentiable at (0, 0).