%% 
%%  An UIT Edition example
%% 
%%  Example 10-04-2 on page 204.
%% 
%%  Copyright (C) 2010 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% 
%% ==== 
% Show page(s) 1
%% 
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{201.70511pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\usepackage{calc}
\StopShownPreambleCommands
\begin{document}
\[
  z =\;\;\underbrace{%
   \makebox[\widthof{~$x^2+y^2$}][r]{%
     $\sqrt{x^2+y^2}$}}_{\scriptstyle=z^2}
\]
\end{document}
