%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 03-02-1 auf Seite 23.
%%  Copyright (C) 2018 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[10pt]{screxa}
\pagestyle{empty}
\setlength\textwidth{170.40707pt}
\setlength\parindent{0pt}
\setcounter{equation}{12}
\renewcommand\theequation{3.\arabic{equation}}
%StartShownPreambleCommands
\makeatletter
\newcommand\be{\@ifstar{\[}{\equation}}
\newcommand\ee{\@ifstar{\]}{\endequation}}
\makeatother
%StopShownPreambleCommands
\begin{document}
\be  f(x)= \int\frac{\sin x}{x}\,\mathrm{d}x \ee
\be* f(x)= \int\frac{\sin x}{x}\,\mathrm{d}x \ee*
\end{document}