Say that f is right-differentiable at x0 if f 0 (x0+) := lim h→0+[ f(x0 +h)− f(x0)]/h exists. (a) Define left-differentiable at x0 and assign a meaning to f 0 (x0−). (b) Show that f is differentiable at x0 if and only if it is both left-differentiable and rightdifferentiable at x0 and f 0 (x0+) = f 0 (x0−