\documentclass{article}
\usepackage[active,tightpage,textmath]{preview}
\usepackage{xepersian}
\settextfont[Scale=1]{Yas}
\setdigitfont{Yas}
\begin{document}
\(\displaystyle\sum_{i=0}^{\infty}\frac{1}{2^i}\)
\end{document}