%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 9-18-2 on page 206.
%% 
%%  Copyright (C) 2009 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.
%% 

\documentclass[]{ttctminimal}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{355.65944pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{30}
\renewcommand\theequation{9.\arabic{equation}}
\StartShownPreambleCommands
\usepackage{amsmath,graphicx}
\newcommand*\Sum[1][1]{\ensuremath\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}
