\documentclass{book}

\usepackage{url}
%\usepackage{fancyvrb}
\usepackage{setspace}\doublespacing
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{makeidx}
\makeindex
\usepackage{float}
\usepackage{xepersian}
%\input{/Users/alikhorramian/Desktop/2015-QFT-09 Oct/BOOK-Field886EA178-B648-4840-A21A-AFFADE5AD3E5.ftx_farsitex_cmds_xepersian.tex}
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\def\d{\displaystyle}
\usepackage{sectsty}
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\begin{document}
\[
L=\frac{1}{2}(\partial _{\mu }\eta )^{2}-\lambda \nu ^{2}\eta ^{3}-\lambda
\nu \eta ^{3}-\frac{1}{4}\lambda \eta ^{4}
\]
\end{document}