\documentclass[a4]{book} 

\usepackage{color}
\usepackage{graphicx}
\usepackage{pict2e} 
\textwidth =12cm 
\textheight =19cm
\usepackage{amsmath,amssymb} 
\usepackage{pict2e} 
\usepackage[font=small,format=plain,labelfont=bf,up,textfont=normal,up , justification=justified,singlelinecheck=false]{caption}
\usepackage{xepersian}
\settextfont[Scale=1]{XB Zar}
\setdigitfont[Scale=1]{XB Zar} 
\begin{document}
\section{مختصات قطبی}
اگر 
$(r,\theta)$
مختصاتِ قطبیِ نقطه‌ی 
$(x,y)$
باشند ، آن‌گاه 
\begin{figure}[!hb]
\setlength{\unitlength}{0.1cm}
\linethickness{0.3mm} 
\begin{LTR}
\begin{picture}(0,20)
\put(0,0){\vector(1,0){40}}
\put(42,0){$x$}
\put(0,0){\vector(0,1){30}}
\put(0,32){$y$}
\put(0,0){\color{red}\vector(2,1){30}} 
\put(30,16){$(x,y)$} 
\put(29,-4){$x$}
\put(-4,16){$y$}
\put(9,1){$\theta$}
\put(14,10){$r$}
\multiput(0,15)(0.8,0){38}{\line(1,0){0.3}} 
\multiput(29.5,0)(0,0.8){20}{\line(0,1){0.3}}
\end{picture} 
\end{LTR} 
\caption {نمایشِ قطبیِ یک نقطه} 
\end{figure}

\end{document}