%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-30 on page II-148 in "breqn --- Automatic line breaking in math displays".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{171.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.8\abovedisplayskip}%
    \setlength\belowdisplayskip{.8\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{breqn}
%StopShownPreambleCommands

\begin{document}
\begin{dmath}
   0 \hiderel{<} x =
   x_0 + x_1 + x_2 + x_3 + \dots + x_{n-1} + x_n
\end{dmath}
\end{document}