\documentclass[hyperref={hyperfootnotes=false}]{beamer}
\setbeamertemplate{navigation symbols}{}
\usefonttheme{serif}
\usepackage[T1]{fontenc}
\usepackage{xepersian}
\settextfont{B Traffic}
\setlatintextfont[Scale=0.8]{Times New Roman}
\setdigitfont{B Traffic}
\DefaultMathsDigits
\begin{document}
\begin{frame}
 $  a^2 = b^2 + c^2$
\end{frame}
\end{document}