Say that f is right-differentiable at xo if f' (xo+) := lim [f(xoth) - f(xo)]/h exists. h - o+ (a) Define left-differentiable at Xo and assign a meaning to f' (xo-). (b) Show that f is differentiable at Xo if and only if it is both left-differentiable and right- differentiable at xo and f' (xo+) = f'(xo-)