突出显示矩阵的顶部和底部

Question

我希望能够制作出如图所示的三角形。希望你能帮助我

Answer 1

这是一种使用方法tikzmark：

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\begin{document}
\begin{equation}
    \hat{A}=\begin{pmatrix}
        a_{11} &\tikzmark{tl}a_{12} & a_{13} & \dots\tikzmark{tr}\\
        \tikzmark{a21}a_{21} & a_{22} & a_{23} & \dots\\
        a_{31} & a_{32} & a_{33} & \dots\tikzmark{dots}\\
        \tikzmark{bl}\vdots & \vdots & \vdots\tikzmark{br} & \ddots
      \end{pmatrix}
\end{equation}
\begin{tikzpicture}[remember picture,overlay]
\draw [blue] ([xshift=-6pt,yshift=-2pt]pic cs:bl) --  ([yshift=10pt]pic cs:a21) -- ([xshift=25pt,yshift=-2pt]pic cs:br) -- cycle;
\draw [red] ([xshift=-10pt,yshift=6pt]pic cs:tl) --  ([yshift=6pt]pic cs:tr) -- ([yshift=-10pt]pic cs:dots) -- cycle;
\end{tikzpicture}
\end{document}

Answer 2

与。{pNiceMatrix}​nicematrix

\documentclass{article}
\usepackage{nicematrix}
\usepackage{tikz}

\begin{document}
\begin{equation}
\hat{A} = 
\begin{pNiceMatrix}
  a_{11} &a_{12} & a_{13} & \dots\\
  a_{21} & a_{22} & a_{23} & \dots\\
  a_{31} & a_{32} & a_{33} & \dots\\
  \vdots & \vdots & \vdots & \ddots
\CodeAfter
  \begin{tikzpicture} [rounded corners]
  \draw [blue] ([xshift=4pt,yshift=3pt]2-|1) 
               |- ([xshift=-4pt,yshift=3pt]last-|last) -- cycle ; 
  \draw [red] ([xshift=-5pt,yshift=-3pt]1-|2) -| ([xshift=-3pt,yshift=-5pt]4-|last) -- cycle ;
  \end{tikzpicture} 
\end{pNiceMatrix}
\end{equation}
\end{document}

由于 PGF/Tikz 节点，您需要进行多次编译。