这是我的 MWE(我想实现括号 A)
\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{mathptmx}
\begin{document}
$\textbf{N}=\begin{pmatrix}
-\dfrac{1}{2} & 0\\
0 & -\dfrac{1}{2}
\end{pmatrix}$------------%%%%%%the output is parentheses B
\end{document}