%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 5-3-3 on page 79.
%% 
%%  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[]{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{146.59192pt}
\setlength\parindent{0pt}
\StartShownPreambleCommands
\usepackage{xcolor,empheq}
\StopShownPreambleCommands
\begin{document}
\begin{empheq}[box=\fcolorbox{blue}{cyan!40}]{align*}
        x(t) & =\frac{a\left(t^{2}-1\right)}{t^2+1}\\
        y(t) & =\frac{at\left(t^{2}-1\right)}{t^2+1}
\end{empheq}
\end{document}