1. [10+6 pts] Let f(x) = x-1+x4 (a) Find f'(x) using the definition of the derivative. No marks will be given if the definition is not used. (b) If f(x) is differentiable at x = a, with a > 0, evaluate the following limit in terms of f'(a): - f(x) = f(a) lim x a x - a