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%%  Ein Beispiel der DANTE-Edition
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%%  2. Auflage
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%%  Beispiel 09-07-27 auf Seite 427.
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%%  Copyright (C) 2016 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.
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%%  See http://www.latex-project.org/lppl.txt for details.
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{169.16928pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{50}
\renewcommand\theequation{9.\arabic{equation}}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\begin{align} \begin{pmatrix}
 1 &-2 & 1 & 2\\ 2 & 3 &-2 & 3\\
 4 &-1 & 3 &-1\\ 3 & 2 &-4 & 5
\end{pmatrix} \end{align}
\end{document}
