乳胶：右方括号

Question

我有一个奇怪的乳胶问题。仅需要从右侧获得方括号，并且对于网络示例，它可以正常工作：

$\left[ 
  \begin{gathered} 
    \left\{ 
      \begin{gathered} 
        x^2 - 6 \geqslant x, \hfill 
        \\ 
        5x < 1 \hfill 
        \\ 
      \end{gathered} 
    \right. \hfill 
    \\ 
    \left\{ 
      \begin{gathered} 
        x^3 - 8 < 0, \hfill 
        \\ 
        x^2 - 7x < 0. \hfill 
        \\ 
      \end{gathered} 
    \right. \hfill 
    \\ 
  \end{gathered} 
\right.$

Result

但是例如\frac{x}{a}无效！

$\left[ 
  \begin{gathered} 
    \frac{2}{3} > 1;
  \\ 
    \frac{2}{3} > 1;
  \end{gathered} 
\right.$

如何获得带分数的方括号？

Answer 1

That both works for me. Maybe you forgot to \usepackage{amsmath}?