Latex：方程组，对齐

1 大花括号：\left\{ 和\right.

2 对齐

\begin{aligned}

&和\\配套，&是需要对齐的位置，\\是换行

\end{aligned}

对齐内容需要包在\begin{aligned}和\end{aligned}里面

k_t^l=\left\{

\begin{aligned}
&{h^{l-1}_t}^TW^l_{h,k}&& if\quad e_t=0,\\
&\frac{1}{2}\left({h^{l-1}_t}^TW^l_{h,k}+E^T_{ent}[e_t]W^l_{e,k}\right) &&else;\\
\end{aligned}
\right.~\tag{3}