LaTeX.org

Thank you very much! ;)

Now it's perfect.

\documentclass[]{article}
\usepackage[brazil]{babel}
\usepackage[latin1]{inputenc}
\usepackage{amsmath,amscd,amsthm}
\usepackage{varwidth}
\pagestyle{empty}
\begin{document}
\center
\begin{varwidth}{1\textwidth}
\center
anything

\[x=a_1+a_2+a_3+a_4+a_5 ...

Hello!

I'd like to know if it's possible to insert a horizontal line whose width is the same as the page content's width.

Example:
\documentclass[]{article}
\usepackage[brazil]{babel}
\usepackage[latin1]{inputenc}
\usepackage{amsmath,amscd,amsthm}
\pagestyle{empty}
\begin{document}
\center ...