%%
%% 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}
