\documentclass{article}
\title{编号测试}
\author{}
\date{}
\usepackage{amsmath}
\usepackage{cases}
\begin{document}
using \{equation\}
\begin{equation}x+y = 1\end{equation}
using \{subequations\}+\{align\}
\begin{subequations}
\begin{align}
x+y=2\\
x-y=2
\end{align}
\end{subequations}
using \{subequations\}+\{equation\}
\begin{subequations}
\begin{equation}x+y = 3\end{equation}
\begin{equation}x+y = 3\end{equation}
\end{subequations}
using \{subnumcases\}
\begin{subnumcases}
{|x|=}
x, &$x\ge 0$,\\
-x, &$x<0$.
\end{subnumcases}
using \{numcases\}
\begin{numcases}{|x|=}
x, & for $x \geq 0$\\
-x, & for $x < 0$
\end{numcases}
\end{document}
