%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-1 on page II-131 in "Display and alignment structures for equations".
%%
%% 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}

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

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

\begin{document}
\begin{equation} (a+b)^2 = a^2+2ab+b^2 \end{equation}
\[  \sin^2\eta+\cos^2\eta = 1  \]
\end{document}
