%%
%% 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
%%
\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}
