Cross partial derivatives with LaTeX

Question

I'm trying to write something simple:

Not quite sure how to write out the second term (from the left) in LaTeX. Any suggestions please?

Answer 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}

There is a difference in the spacing between using a \biggl(...\biggr) and \left(...\right) pair. Your choice, although the former is preferred.