答案1
\documentclass{article}
\begin{document}
\[
\frac{\partial^2 f}{\partial y \, \partial x}
= \frac{\partial}{\partial y} \biggl( \frac{\partial f}{\partial x} \biggr)
% = \frac{\partial}{\partial y} \left( \frac{\partial f}{\partial x} \right)
= (f_x)_y
= f_{xy}
= \partial_{yx} f
\]
\end{document}
\biggl(
使用...\biggr)
和\left(
...对之间的间距有所不同\right)
。您可以选择,尽管前者是首选。