% !TeX program = xelatex
\documentclass[12pt]{book}
\usepackage[paperheight=23cm, paperwidth=14.8cm, left=1.4cm, right=1.4cm, top=2cm, bottom=1.5cm, includehead=true,includefoot=true, centering]{geometry}
\usepackage{amsthm,amsmath,amssymb}
\usepackage[extrafootnotefeatures]{xepersian}
\settextfont{Nazli}
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\setlatintextfont[Scale=0.86]{Times New Roman}

\begin{document}

\begin{equation}
a=b+c
\end{equation}

\end{document}