测试

\begin{align}
& \frac{{ - b \pm \sqrt {{b^2} - 4ac} }}{{2a}} \\\\
& \frac{{\sqrt {{c^2} - {d^2}} }}{{3a}}
\end{align}

显示如下

\begin{align}
& \frac{{ - b \pm \sqrt {{b^2} - 4ac} }}{{2a}} \\
& \frac{{\sqrt {{c^2} - {d^2}} }}{{3a}}
\end{align}