Def: Let f be a real-valued function defined on an interval I containing the point c, (we allow the possibilty that c is an endpoint of I) we say that f is differentiable at c (or has a derivative at c) if the limit

lim x->c (f(x) - f(c))/(x-c)

exists and is finite.

A) Use definition above to prove that f'(x) = (1/3)^(-2/3) for x is not equal to 0

B) Show that f is not differentiable at x = 0