\documentclass[12pt,a4paper]{book}
\usepackage{amsmath,amssymb,amsthm}
\usepackage{graphicx,subfigure,tikz}



%\usepackage[pagebackref=true,colorlinks,linkcolor=black,citecolor=magenta]{hyperref}
\usepackage{xepersian}
\begin{document}


\begin{figure}[h]
\begin{center}
\begin{tikzpicture}
\begin{scope}[>=latex]
\draw[fill=gray!15!white] (0,0) -- (3.5,0.7) -- (4.7,2.62) -- (1.2,1.92) -- (0,0);
\draw[->,thick] (0,0) -- (3.5,0.7);
\draw[->,thick] (0,0) -- (1.2,1.92);
\draw[very thick][densely dashed] (1.2,1.92) -- (1.2,0.24); 
\draw [<->] (0.7,0.14) arc (0:40:1cm);
\draw (1.2, 1) node[right]{\scalebox{0.6}{$\parallel B\parallel \sin{\theta}$}};
\draw (0.2, 0.3) node[right]{\scalebox{0.9}{$\theta$}};
\draw[->,thick] (4.6, 1.5) -- (3.3, 1.5);
\draw (4.6,1.5) node[right]{\scalebox{0.7}{\RTL{=مساحت$\parallel A\times B\parallel$}}};
\end{scope}
\end{tikzpicture}
\caption{اندازه $A\times B$ برابر است با مساحت متوازی‌الاضلاع ساخته شده روی $A$ و $B$.}\label{fig2.5}
\end{center}
\end{figure}

\end{document}