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*}