%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 09-24-2 auf Seite 206.
%%  Copyright (C) 2018 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.
%% 
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% Show page(s) 1
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\documentclass[10pt]{screxa}
\pagestyle{empty}
\setlength\textwidth{352.81416pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{30}
\renewcommand\theequation{9.\arabic{equation}}
%StartShownPreambleCommands
\usepackage{amsmath,graphicx} \newcommand*\Sum[1][1]{\mathop{\scalebox{#1}{$\displaystyle\sum$}}}
%StopShownPreambleCommands
\begin{document}
\begin{align}
 \Sum_{i=n}^ma_i\cdot\sum_{j=l}^kb_j &= \Sum[1.2]_{i=n}^m\Sum_{j=l}^ka_ib_j
\end{align}
\end{document}