谢谢你的解答

documentclass[12pt,UTF8]{article}
usepackage[space,fntef]{ctex}
usepackage[english]{babel}
usepackage{amsmath,amsthm}
usepackage{amsfonts}
usepackage{txfonts}
usepackage[Symbol]{upgreek}
letpiuppi
letleleqslant

begin{document}

[frac{1}{2}] [frac{x^2}{a^2}+frac{y^2}{b^2}=1]
[frac{3sqrt{3}}{5}] [frac{3pi}{2}]
end{document}

\documentclass[UFT8]{ctexart}
\usepackage{tikz}
\usepackage{amsmath,forest}
\usetikzlibrary{decorations.pathreplacing}
\forestset{
  forest scheme/.style={
    for tree={
      child anchor=west,
      parent anchor=east,
      grow'=0,
      anchor=west,
      align=left,
      if n=1{%
        edge path={
          \noexpand\path [\forestoption{edge}] ([yshift=0.7em]!ul.south west) -- ([yshift=-0.7em]!u1.north west)\forestoption{edge label};
        }
      }{no edge},
      edge={decorate, decoration={brace,amplitude=2mm,raise=3pt}},
    },
  }
}
\begin{document}
\begin{forest}
forest scheme
  [Main
    [第二级1
      [第三级11]
      [第三级12]
    ]
    [第二级2
      [第三级21
      \vspace*{15pt} [\parbox{6em}{文字文字文字文字文字文字文字文文字}]
      [结构]
      ]
      [第三级22]
    ]
  ]
\end{forest}
\end{document}