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%%  Ein Beispiel der DANTE-Edition
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%%  2. Auflage
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%%  Beispiel 09-08-41 auf Seite 466.
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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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% Show page(s) 1
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{193.16928pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{99}
\renewcommand\theequation{9.\arabic{equation}}
%StartShownPreambleCommands
\usepackage{mathtools}
\newcounter{linecnt}
\renewcommand\thelinecnt{(\alph{linecnt})}
\newcommand\stepline{%
  \stepcounter{linecnt}\thelinecnt}
\newgathered{agathered}
  {\llap{\stepline}\quad\hfil}{\hfil}
  {\setcounter{linecnt}{0}}
%StopShownPreambleCommands
\begin{document}
\begin{align}\begin{agathered}
 i_{11}+i_{21}=0.25\\ i_{21}=\frac{1}{3}i_{11}\\
 i_{31}=0.33i_{22}+i_{11}-0.1i_{21}
\end{agathered}\\[7pt]
\begin{agathered}
 f(x)=3x+4\\  g(x)=\sqrt{x^2-1}
\end{agathered}\end{align}
\end{document}