Originally Posted by

**jonmondalson** Hello,

I have to find the derivative of a general function $\displaystyle f(x,y)$ where $\displaystyle x(s,t) = \frac{s+t}{2}$ and $\displaystyle y(s,t) = \frac{s-t}{2}$.

I have already determined that:

$\displaystyle \frac{\partial f}{\partial s} = \frac{1}{2}(\frac{\partial f}{\partial x} + \frac{\partial f}{\partial y})$

and

$\displaystyle \frac{\partial f}{\partial t} = \frac{1}{2}(\frac{\partial f}{\partial x} - \frac{\partial f}{\partial y})$

So, my question is, how can I use this information to get a value for $\displaystyle \frac{\partial^2f}{\partial s \partial t}$, again for a general function?