%% 
%%  An UIT Edition example
%% 
%%  Example 05-03-3 on page 74.
%% 
%%  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{375.57637pt}
\setlength\parindent{0pt}
\setcounter{equation}{1}
\renewcommand\theequation{5.\arabic{equation}}
\StartShownPreambleCommands
\usepackage{xcolor,amsmath,calc}
\StopShownPreambleCommands
\begin{document}
\colorbox{black!20}{%
\parbox{\linewidth-2\fboxsep}{%
  \begin{align}
    \frac{I(\alpha )}{I_{0}}=
    \begin{cases}
      \sqrt{1-\frac{\alpha}{\pi}+\frac{1}{2\pi}\sin 2\alpha} & \varphi =0\\[0.4cm]
      \sqrt{2\left(1-\frac{\alpha}{\pi}\right)\left(2+\cos 2\alpha \right)+
         \frac{3}{\pi}\sin 2\alpha} & \varphi =\frac{\pi}{2}
    \end{cases}
\end{align}}}
\end{document}