%% 
%%  An UIT Edition example
%% 
%%  Example 02-02-1 on page 6.
%% 
%%  Copyright (C) 2010 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.
%% 
%% 
%% ==== 
% Show page(s) 1
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{177.70511pt}
\setlength\parindent{0pt}
\usepackage{setspace}\doublespacing
\StartShownPreambleCommands

\StopShownPreambleCommands
\begin{document}
\( \sum_{i=1}^{n}i=\frac{1}{2}\,n\times(n+1) \)

\begin{math}
  \sum_{i=1}^{n}i=\frac{1}{2}\,n\times(n+1)
\end{math}

$ \sum_{i=1}^{n}i=\frac{1}{2}\,n\times(n+1) $

\end{document}