\documentclass[12pt,a4paper]{article}
\usepackage[francais]{babel}
\usepackage{mafr}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{lmodern}
\usepackage[dvipsnames]{xcolor}
\usepackage{pstricks}
\usepackage{pst-node}
\begin{document}
\psset{arrowscale=1.7}
\begin{pspicture}(-0.25,-0.5)(2.5,1.5)
\pspolygon(0,0)(2,0)(2,1)%
\pcline[arrows=<->,offset=-8pt](0,0)(2,0)\ncput*{$a$}%
\pcline[arrows=<->,offset=-8pt](2,0)(2,1)\ncput*[nrot=90]{$b$}%
\pcline[arrows=<->,offset=8pt](0,0)(2,1)\ncput*[nrot=:U]{$c$}%
\end{pspicture}
\end{document}
