\documentclass{report}

\usepackage{amsthm,amsmath,amssymb}

\newtheorem{defe}{تعریف}[section]
\newtheorem{theorem}{قضیه}[section]
\newtheorem{lemma}{لم}[section]
\newtheorem{proposition}{گزاره}[section]
\usepackage{xepersian}
\begin{document}
\begin{equation}
h_1(x) =\int_{a}^{x}(x-t)g(t)\;dt+\frac{(a-x)}{(b-a)+\lambda(\xi - a)}\left[ \int_{a}^{b}(b-t)g(t)\;dt+\lambda \int_{a}^{\xi}(\xi-t)g(t)\;dt \right] +\frac{(b-x)+\lambda(\xi -x)}{(b-a)+\lambda(\xi - a)}\alpha +\frac{(x-a)}{(b-a)+\lambda(\xi - a)}\beta.
\end{equation}
\end{document}