%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 3-3-1 on page 22.
%% 
%%  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{170.59192pt}
\setlength\parindent{0pt}
\setcounter{section}{3}
\setcounter{equation}{13}
\renewcommand\theequation{3.\arabic{equation}}
\StartShownPreambleCommands
\newcommand*\diff{\mathop{}\!\mathrm{d}}
\StopShownPreambleCommands
\begin{document}
\begin{equation}
f(x)= \int\frac{\sin x}{x}\diff x \end{equation}
\renewcommand*\theequation{%
  \thesection-\Alph{equation}}
\begin{equation}
f(x)= \int\frac{\sin x}{x}\diff x
\end{equation}
\end{document}