\documentclass[11pt]{article}
\usepackage[brazil]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[top=4.5cm,left=4.5cm,right=4.5cm,bottom=4.5cm]{geometry}
\usepackage{graphicx}
\usepackage{amsthm}
\usepackage{amsfonts,amssymb,amsxtra,empheq}
\usepackage[mathscr]{eucal}
\begin{document}
<<fig=T,echo=F>>=
plot(1:10,1:10)
text(3,8,expression('se x \u2264 3'))
@
\end{document}