If $f:R\rightarrow R$ is defined as $f(x)=x^2+1$ then $f^{-1}(x)$ = ?

Question

$\begin{array}{1 1} \large\frac{1}{x^2+1} \\ \sqrt {x-1} \\ f^{-1}(x) \text{does not exist because f is not 1-1.} \\f^{-1}(x) \text{ does not exist because f is not onto.} \end{array}$