如何在 dcases 中添加 alignat？

Question

我正在尝试

我试过

\documentclass[12pt,a4paper]{article}
\usepackage{mathtools}
\begin{document}
    \begin{alignat*}{2}
        2 x_{2}&=x_{1}&&+\frac{2}{x_{1}},\\
        2 x_{3}&=x_{2}&&+\frac{2}{x_{2}},\\
        \MTFlushSpaceAbove
        &\vdotswithin{+} && \vdotswithin{+}
        \MTFlushSpaceBelow
        2 x_{n}&=x_{n-1}&&+\frac{2}{x_{n-1}},\\
        2 x_{1}&=x_{n}&&+\frac{2}{x_{n}}.
    \end{alignat*}
\end{document}

我只得到

如何得到结果？

Answer 1

这里有两种可能的解决方案。第二种需要加载empheq（加载mathtools）：

    \documentclass[12pt,a4paper]{article}
    \usepackage{empheq}

    \begin{document}

    \[ \begin{dcases}
    \begin{alignedat}{2}
            2 x_{2}&=x_{1}&&+\frac{2}{x_{1}},\\
            2 x_{3}&=x_{2}&&+\frac{2}{x_{2}},\\
            \MTFlushSpaceAbove
            &\vdotswithin{+} && \vdotswithin{+}
            \MTFlushSpaceBelow
            2 x_{n}&=x_{n-1}&&+\frac{2}{x_{n-1}},\\
            2 x_{1}&=x_{n}&&+\frac{2}{x_{n}}.
        \end{alignedat}
        \end{dcases}\]
    \bigskip

    \begin{empheq}[left=\empheqlbrace]{alignat*=2}
            2 x_{2}&=x_{1}&&+\frac{2}{x_{1}},\\
            2 x_{3}&=x_{2}&&+\frac{2}{x_{2}},\\
            \MTFlushSpaceAbove
            &\vdotswithin{+} && \vdotswithin{+}
            \MTFlushSpaceBelow
            2 x_{n}&=x_{n-1}&&+\frac{2}{x_{n-1}},\\
            2 x_{1}&=x_{n}&&+\frac{2}{x_{n}}.
        \end{empheq}

    \end{document} 

Answer 2

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation*}
  \left\{ 
    \begin{alignedat}{2}
       2 x_{2}&=x_{1}&&+\frac{2}{x_{1}},\\
       2 x_{3}&=x_{2}&&+\frac{2}{x_{2}},\\
       \MTFlushSpaceAbove
       &\vdotswithin{+} && \vdotswithin{+}
       \MTFlushSpaceBelow
       2 x_{n}&=x_{n-1}&&+\frac{2}{x_{n-1}},\\
       2 x_{1}&=x_{n}&&+\frac{2}{x_{n}}.
    \end{alignedat}
  \right.
\end{equation*}
\end{document}