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\documentclass[12pt]{ctexart}
\usepackage{amsmath}
\usepackage{empheq}
\usepackage{bm}
\begin{document}
\begin{equation}
a^2+b^2=c^2
\end{equation}
\begin{empheq}[left={a=\empheqlbrace}]{align}
\dot{\bm{\lambda}} (t) &= -\frac{\partial H}{\partial \bm{x}}, \label{AA} \\
\frac{\partial H}{\partial \bm{u}} &= 0, \label{BB} \\
\dot{\bm{x}} &= \bm{f} (t,\bm{x}, \bm{u}), \notag
\end{empheq}
\begin{equation}
a^2+b^2=c^2
\end{equation}
\end{document}




























问 怎样在保证在=处对齐的前提下给部分有编号的方程组加左花括号