%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-32 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{159.0pt}

%StartShownPreambleCommands
\usepackage{breqn}
%StopShownPreambleCommands

\begin{document}
A bad result:
\begin{dmath}
  (a+b)^2 = a^2 +2ab + b^2  % A \\ is not helping;
                            % see explanation above!
  (a-b)^2 = a^2 -2ab + b^2
\end{dmath}
\end{document}