<html><head><meta name="color-scheme" content="light dark"></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-4-28 on page II-175 in "amsmath, mathtools, empheq --- Boxed formulas".
%%
%% 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{183.0pt}

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

%StartShownPreambleCommands
\usepackage{empheq,xcolor}
%StopShownPreambleCommands

\begin{document}
\begin{empheq}[box=\fcolorbox{black}{blue!10},
  innerbox=\colorbox{white},
  left=\longrightarrow \empheqlbrace,
  right=\empheqbigrbrack \Longrightarrow]{align}
    x_0&amp;=1     &amp;  x_1&amp;=2   \\
    x_2&amp;=3     &amp;  x_3&amp;=5   \\
    x_4&amp;=8     &amp;  x_5&amp;=13  \\
    x_6&amp;=21    &amp;  x_7&amp;=34  \\
       &amp;\ldots &amp; \sum_{i=1}^n x_i
                    &amp;=\mathord{??}  \tag{*}
\end{empheq}
\end{document}
</pre></body></html>