%% 
%%  Example 02-18-2 
%%  Copyright (C) 2016 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\fbox{%
\begin{minipage}{10cm}
\begin{align*}
  a&=b \\ c&=d
\end{align*}
\end{minipage}}

\noindent\fbox{%
\begin{minipage}{10cm}
\noindent\begin{align*}
  a&=b \\ c&=d
\end{align*}
\end{minipage}}
\end{document}
