%%
%% 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&=1 & x_1&=2 \\
x_2&=3 & x_3&=5 \\
x_4&=8 & x_5&=13 \\
x_6&=21 & x_7&=34 \\
&\ldots & \sum_{i=1}^n x_i
&=\mathord{??} \tag{*}
\end{empheq}
\end{document}
