%% 
%%  An UIT Edition example
%% 
%%  Example 10-07-5 on page 212.
%% 
%%  Copyright (C) 2010 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[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{177.70511pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\newsavebox\lW \sbox\lW{$P_{3}+P_{2}-P_{1}$}
\StopShownPreambleCommands
\begin{document}
\begin{itemize}
\item first function: \\
  $\makebox[\wd\lW][r]{$P_1$} =
    \displaystyle\sum_a \in A$
\item another function: \\
  $\makebox[\wd\lW][r]{%
    $\sin\left(P_1\right)$} = \mathrm{const.}$
\item and a bit different: \\
  $P_{3}+P_{2}-P_{1} = 0$
\end{itemize}
\end{document}