demo
\[ \begin{align}\begin{aligned}(a + b)^2 = a^2 + 2ab + b^2\\(a - b)^2 = a^2 - 2ab + b^2\end{aligned}\end{align} \]
\[\begin{split}\begin{equation}\label{xx}
\begin{split}
a& =b+c-d\\
& \quad +e-f\\
& =g+h\\
& =i
\end{split}
\end{equation}\end{split}\]
https://iridescent.ink/HowToMakeDocs/Basic/SphinxExtensions.html