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%%  Example 07-02-2 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
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\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
%StartShownPreambleCommands

%StopShownPreambleCommands
\begin{document}
\[
\begin{array}{l@{\:=\:}*{5}{l@{\:+\:}}l}
  y_1 & a_{11}x_1 & a_{12}x_2 & a_{13}x_3 & \dots & a_{1(n-1)}x_{n-1} & a_{1n}x_n \\
  y_2 & a_{21}x_1 & a_{22}x_2 & a_{23}x_3 & \dots & a_{2(n-1)}x_{n-1} & a_{2n}x_n \\
  \ \vdots &\ \vdots &\ \vdots &\ \vdots &\ \vdots &\ \vdots &\ \vdots\\
  y_{n-1} & a_{(n-1)1}x_1 & a_{(n-1)2}x_2 & a_{(n-1)3}x_3 & \dots
                                           & a_{(n-1)(n-1)}x_{n-1} & a_{(n-1)n}x_n\\
  y_n & a_{n1}x_1 & a_{n2}x_2 & a_{n3}x_3 & \dots & a_{(n)(n-1)}x_{n-1} & a_{nn}x_n
\end{array}
\]
\end{document}
