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%%  Ein Beispiel der DANTE-Edition
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%%  1. Auflage
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%%  Beispiel 09-08-46 auf Seite 494.
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%%  Copyright (C) 2011 Herbert Voss
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%%  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.
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% Show page(s) 1
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{118.324pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\usepackage{mathtools}
\DeclarePairedDelimiter\abs{\lvert}{\rvert}
\StopShownPreambleCommands
\begin{document}
\[ \abs{a} ~ \abs{\frac{a}{b}} ~ \abs*{\frac{a}{b}} \]
\[ \abs[\big]{\frac{a}{b}} ~ \abs[\bigg]{\frac{a}{b}} \]
\end{document}