%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 8-1-12 on page 162.
%% 
%%  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[]{ttctminimal}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{355.65944pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\newcommand*\diff{\mathop{}\!\mathrm{d}}
\newlength\PREsave
\StopShownPreambleCommands
\begin{document}
Die effektive \ldots
\[
  \fbox{\the\predisplaysize}\quad F(x)=\int f(x)\diff x
  \global\PREsave=\predisplaysize
\]
\advance\PREsave by -2em\rule{\PREsave}{1pt}
\end{document}