%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 03-03-2 auf Seite 25.
%%  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
%% 
%% 
\documentclass[10pt]{exaartplain}
\pagestyle{empty}
\setlength\textwidth{170.40707pt}
\setlength\parindent{0pt}
\setcounter{equation}{15}
\renewcommand\theequation{3.\arabic{equation}}
%StartShownPreambleCommands
\makeatletter
\renewcommand*\@eqnnum{{%
  \normalfont\normalcolor[\theequation]}}
\makeatother
\newcommand*\diff{\mathop{}\!\mathrm{d}}
%StopShownPreambleCommands
\begin{document}
\begin{equation}
f(x)= \int\frac{\sin x}{x}\diff x
\end{equation}
\end{document}