\documentclass[12pt]{book}
\usepackage{amsmath}
\let\orimathchoice\mathchoice
\usepackage{mathstyle}
\begin{document}
\[
\frac{\frac{f\sqrt[x]{2} \sqrt{2}}{2}}{xxx}
\]
\let\mathchoice\orimathchoice
\[
\frac{\frac{f\sqrt[x]{2} \sqrt{2}}{2}}{xxx}
\]
\end{document}