%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-8 on page II-136 in "A single equation on several lines: with alignment".
%%
%% 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{207.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.5\abovedisplayskip}%
    \setlength\belowdisplayskip{.5\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage[tbtags]{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{equation}
  \begin{split}
  (a + b)^3 &= (a + b) (a + b)^2        \\
            &= (a + b)(a^2 + 2ab + b^2) \\
            &= a^3 + 3a^2b + 3ab^2 + b^3
  \end{split}
\end{equation}
\end{document}