What properties do I have if I know $f$ and $f^{-1}$inverse are differentiable?

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My goal is to show that $(f^{-1})'(y) = 1/[f'(f^{-1}(y)]$ for all $y$ in $(a,b)$. I have no idea where to start. I know that $f^{-1}$ and $f$ are differentiable.

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You have that

$$(f^{-1}\circ f)(x)=x.$$ Taking derivatives on both sides and applying the chain rule on the left side we get

$$(f^{-1})'(f(x))\cdot f'(x)=1,$$ from where

$$(f^{-1})'(f(x))=\frac{1}{f'(x)}.$$

Writing $y=f(x)$ (and thus $x=f^{-1}(y)$) it is

$$(f^{-1})'(y)=\frac{1}{f'(f^{-1}(y))}.$$