The following is paraphrased from Stewart's Calculus text.

Theorem: If is a one-to-one differentiable function with inverse function and , then the inverse function is differentiable at and

Proof: We have . Since and are inverses we have iff and iff .

Since is differentiable, it is continuous, so is continuous by earlier theorem. Thus as we have , or . Therefore:

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The starred step is the one I'm having difficulty with understanding fully. I understand the continuity argument that as we have , but it seems like a bit of a leap, at least I can't think of any definitions or theorems to apply, to use that to change to (although an argument seems to work in the opposite direction--that would imply that ). It seems that the claim in Stewart's argument also has to do with the fact that and are one-to-one. Any thoughts?