maybe related:
- https://ask.latexstudio.net/ask/question/8093.html
- https://tex.stackexchange.com/questions/724439/aligning-equations-inside-cases-with-individual-text-tags-in-each-row-and-equa
矛盾点在于：
- 「
\left\{和\right.」要被视为一个整体 - 「
equation x」与「a,b,c」要被视为两个(列)部分
不是很优雅的做法，并没有解决上面的矛盾，借助了tag...
\documentclass[12pt]{ctexart}
\usepackage{showframe}
\usepackage{amsmath,amsfonts}
\usepackage{empheq}
\usepackage{lipsum}
\usepackage{mathtools}
\begin{document}
balabala
\begin{equation}
a^2+b^2=c^2
\end{equation}
\lipsum[2]
\newtagform{myempty}{}{}
\usetagform{myempty}
\begin{empheq}[left=\empheqlbrace]{align}
a^2+b^2=c^2, \tag{a} \\
a^2+b^2=c^2, \tag{b} \\
a^2+b^2=c^2, \tag{c}
\end{empheq}
\usetagform{default}%
\lipsum[2]
\begin{equation}
a^2+b^2=c^2
\end{equation}
\end{document} 或者更优雅一点写成：
\documentclass[12pt]{ctexart}
\usepackage{showframe}
\usepackage{amsmath,amsfonts}
\usepackage{empheq}
\usepackage{lipsum}
\usepackage{mathtools}
\newtagform{myempty}{}{}
\AddToHook{env/empheq/before}{\usetagform{myempty}}
\AddToHook{env/empheq/after}{\usetagform{default}}
\begin{document}
balabala
\begin{equation}
a^2+b^2=c^2
\end{equation}
\lipsum[2]
\begin{empheq}[left=\empheqlbrace]{align}
a^2+b^2=c^2, \tag{a} \\
a^2+b^2=c^2, \tag{b} \\
a^2+b^2=c^2, \tag{c}
\end{empheq}
\lipsum[2]
\begin{equation}
a^2+b^2=c^2
\end{equation}
\end{document} 
问 如何让自定义标签右对齐？