%% 
%%  An UIT Edition example
%% 
%%  Example 10-04-1 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{153.70511pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands

\StopShownPreambleCommands
\begin{document}
\[ z=\sqrt{\underbrace{x^2+y^2}_{\scriptstyle=z^2}}\]
\[ z=\underbrace{\sqrt{x^2+y^2}}_{\scriptstyle=z^2}\]
\end{document}