Theorem 7.2.1 Derivatives of Inverse Functions
Let f be differentiable and one-to-one on an open interval I, where f'(x) ≠ 0 for all x in I, let J be the range of f on I, let g be the inverse function of f, and let f(a) = b for some a in I. Then g is a differentiable function on J, and in particular,
(f^(-1))'(b) = g'(b) = (1) / (f'(a))
(f^(-1))'(x) = g'(x) = (1) / (f'(g(x)))