Decide if the following statements are true or false. If true, give a proof. If false, provide a counter-example (with an explanation). (a) If f + g and f are differentiable at a, then g is differentiable at a. (b) If f . g and f are differentiable at a, then g is differentiable at a. (c) If fog and g are differentiable at a, then f is differentialbe at g(a)