Use the theorem in Sec. 23 to show that each of these functions is differentiable in the indicated domain of definition, and also to find f'(z):
(a) f (z) = 1/z4 (z ≠ 0);
(b) f (z) = √reiθ/2 (r > 0, α < θ < α + 2π);
(c) f (z) = e−θ cos(ln r) + ie−θ sin(ln r) (r > 0, 0 < θ < 2π).