# Determinant of 3x3 matrix by cofactor expansion

A simple code with pstricks and mathtools:

\documentclass[svgnames]{article}
\usepackage{mathtools}
\usepackage{pst-node, auto-pst-pdf}

\begin{document}

\begin{postscript}
\psset{{linecolor=DeepPink}, framesep=2pt, nodesepB=-8pt}
$\begin{bmatrix} \mathclap{\circlenode[linestyle=dashed, dash=3.5pt 2.5pt]{A}{0}} & 2 & \rnode{C}{1} \\ 3 & -1 & 2 \\ \rnode{B}{4} & 0 & 1 \end{bmatrix}$
\ncline{A}{B}
\ncline{A}{C}
\end{postscript}

\end{document}


\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fit}

\newcommand{\tikzmark}[2]{\tikz[overlay,remember picture,baseline=(#1.base)] \node (#1) {#2};}

\begin{document}
$\left[\begin{array}{*3{c}} \tikzmark{left_end}{0} & 2 & \tikzmark{right_end}{1}\\ 3 & -1 & 2 \\ \tikzmark{down_left}{4} & 0 & \tikzmark{down}{1} \end{array}\right]$
\tikz[overlay,remember picture] {
\draw[red,thick,dashed] (left_end) circle (0.2cm);
\draw[-,red,thick] (left_end) -- ++ (1.7,0) (right_end);
\draw[-,red,thick] (left_end) -- ++ (0,-1.) (down_left);
}
\end{document}


• I posted some extended comments below. If you address them in your answer, I will be happy to remove them. – marmot Feb 22 at 16:00
• Thanks @marmot. Interesting comments and smoother solution. – Sina Ahmadi Feb 22 at 16:05
• You can really take them over, I do not want to post an answer here. I just want to avoid that \tikzmark gets redefined. – marmot Feb 22 at 16:07

This is more an extended comment on the answer by Sina Ahmadi. I will be happy to remove this if in that answer a few things get changed.

1. \tikzmark is a phantastic command that is part of the tikzmark library. I would like to argue that creating a new command with this name is not a good practice.
2. The nodes right_end and down_left are not used. The lines are drawn as determined by the two hard-coded distances - ++ (1.7,0) and - ++ (0,-1.).

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

\begin{document}
$\begin{bmatrix} \tikzmarknode[circle,draw=red,inner sep=0.5pt,densely dashed]{A11}{0} & 2 & \tikzmarknode{A13}{1}\\ 3 & -1 & 2 \\ \tikzmarknode{A31}{4} & 0 & 1 \end{bmatrix}$
\begin{tikzpicture}[overlay,remember picture]
\draw[red] (A11.east) -- (A11.east-|A13.east)
(A11.south) -- (A11.south|-A31.south);
\end{tikzpicture}
\end{document}


• @Said You're welcome! (BTW, you can only accept one answer. Given that you accepted Sina's answer first, you might want to place the tick there.) – marmot Feb 22 at 22:04