1. 是aligned，不是align！！

网上写的公式大多是这样的

\begin{equation}\label{eq:2}
\begin{align} 
Q\left( {s,t} \right) = a{s^2} + 2bst + c{t^2} + 2ds + 2et + f
\end{align}
\end{equation}

但是报错：

! Package amsmath Error: Erroneous nesting of equation structures;
(amsmath)                trying to recover with `aligned'.
！ 包amsmath错误:方程结构嵌套错误;
(amsmath)尝试用`aligned'恢复。
See the amsmath package documentation for explanation.
Type  H <return>  for immediate help.

align换成aligned，不报错了

\begin{equation}\label{eq:2}
\begin{aligned} 
Q\left( {s,t} \right) = a{s^2} + 2bst + c{t^2} + 2ds + 2et + f
\end{aligned}
\end{equation}

2. \begin{document}

重新运行报错

Runaway argument?
{2 
! File ended while scanning use of \@LN.
<inserted text> 
                \par 
l.30 \begin{document}
                     
? 

解决办法：关掉所有编译后，
选择：文件→删除辅助文件→选择所有→删除。
再次运行就可以了