答案1
最后我按照以下方式进行操作:
\documentclass[12pt]{article}
% create boxes:
\usepackage{tikz}
\usepackage[most]{tcolorbox}
\tcbset{enhanced,boxrule=0pt, colback=white,
borderline north={1mm}{0mm}{black},
borderline south={1mm}{0mm}{black},
borderline west ={1mm}{2mm}{black}
}
\begin{document}
This is my calculation:
\begin{align}
A=A_1+A_2
\end{align}
\begin{tcolorbox}
We use:
\begin{align*}
A_1&=2\pi r^2\\
A_2&=l^2
\end{align*}
\end{tcolorbox}
\begin{align}
A=2\pi r^2+l^2
\end{align}
\end{document}
这真是太好了!