\documentclass{article}
\usepackage[width=17cm,right=2cm,height=24cm,top=2cm]{geometry}
\usepackage{unicode-math}
\usepackage{amsmath}
\usepackage{xepersian}
\settextfont[Scale=1.5]{XB Yas}
\setdigitfont[Scale=1.2]{XB Yas}
\setlatintextfont[Scale=1.5]{Times New Roman}
\setmathfont[Scale=1.5]{latinmodern-math.otf}
\linespread{2}
\begin{document}
$\frac{a_n+a_{n+1}+\dots+a_{n+s-1}}{k}$
\end{document}