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%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 09-26-2 auf Seite 207.
%%  Copyright (C) 2018 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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\documentclass[10pt]{exaartplain}
\pagestyle{empty}
\setlength\textwidth{352.81416pt}
\AtBeginDocument{\setlength\parindent{0pt}}
%StartShownPreambleCommands
\usepackage{witharrows}  \WithArrowsOptions{displaystyle,tikz=blue}
%StopShownPreambleCommands
\begin{document}

$\begin{WithArrows}
\sum_{i=1}^n (x_i+1)^2 &= \sum_{i=1}^n (x_i^2+2x_i+1) \Arrow{by linearity}\\
                       &= \sum_{i=1}^n x_i^2 + 2\sum_{i=1}^nx_i+ n
\end{WithArrows}$
\end{document}