Let $I \subseteq \Bbb R$ be a non-degenerate open interval, and let $f:I\rightarrow \Bbb R$ be a function. Suppose that f is strictly monotone. If f is continuous and strictly increasing (or decreasing), then the function $f^{-1}:f(I)\rightarrow I$ is continuous and strictly increasing (or decreasing).
Strictly monotone means the function is either strictly increasing or strictly decreasing.
Strictly increasing means that $x<y$ implies $f(x)<f(y)$ for all $x,y\in A$.
Strictly decreasing means that $x<y$ implies $f(x)>f(y)$ for all $x,y\in A$.
Now for my attempt:
Suppose that $f^{-1}(x)>0$ for all x in the interior of I. Let $x,y\in I$. Suppose that $x<y$. Then $[x,y]\subseteq I$. The Mean Value Theorem applied to $f\rvert [x,y]$ implies that there is some $c\in (x,y)$ such that $f^{-1}(c)=$${f(y)-f(x)}\over {y-x}$.
Because $c\in (x,y)$ then c is in the interior of I, and it follows that $f(y)-f(x)=f^{-1}(c)(y-x)>0$. Therefore $f(x)<f(y)$, and f is increasing.
Best Answer
First we will show $f^{-1}$ is strictly increasing:
Thus $f^{-1}$ is strictly increasing as required.
Now to show $f^{-1}$ is continuous: