\documentclass[a4paper,12pt]{report}
\usepackage{graphicx}  % inserting images
% setting the margins of page
% \usepackage[top=3cm,right=3cm,bottom=2.5cm,left=2.5cm]{geometry}                 
\usepackage[colorlinks,linkcolor=blue,citecolor=red]{hyperref}  % PDF links
% for switching between double/single space in document
% package for Headings
\usepackage{xepersian}
%\usepackage{amsmath }
\usepackage{amsthm,amssymb,amsmath}
\renewcommand*{\baselinestretch}{1.8}
\settextfont{XB Zar}
\setdigitfont{XB Zar}
\defpersianfont\kayhan{XB Kayhan}
\usepackage{ wasysym }
\newtheorem{theorem}{قضیه}
\theoremstyle{plain}
\newtheorem{lemma}{لم}
 \newtheorem{proposition{گزاره} 
\theoremstyle{definition} 
\newtheorem{definition}{تعریف} 
\newtheorem{example}{مثال} 
\newtheorem{prob}{سوال} 
\theoremstyle{remark} 
\newtheorem{corollary}{نتیجه} 
\newtheorem{remark}{ملاحظه}
\begin{document}

 \begin{equation}
 \frac{\phi(t)-\phi(s)}{t-s}\leq \frac{\phi(u)-\phi(t)}{u-t}
 \end{equation}
\end{document}