%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-7-20 on page II-206 in "resizegather --- Downscaling an equation".
%%
%% 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{127.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.2\abovedisplayskip}%
    \setlength\belowdisplayskip{.2\belowdisplayskip}%
  }

%StartShownPreambleCommands
     \usepackage[warningthreshold=.1]{resizegather}
%StopShownPreambleCommands

\begin{document}
     \begin{gather} a+b+c+d+e+f+g   = X \\
                    a+b+c+d+e+f+g+h = Y \end{gather}
\end{document}