latex之插入数学公式

1.

\begin{eqnarray}
Ly = \lambda y
\end{eqnarray}

2.

\begin{gather*}
\begin{split}
b=1\\
a=b
\end{split}\\[6pt]
\end{gather*}

3.

\begin{gather*}
\begin{split}
a &= m\\
   &= k
\end{split}\\[6pt]
\begin{align}
b &= i ,\\
c &= j
\end{align}
\end{gather*}