Possible Duplicate:
How does partial derivative work?
If $\frac{\partial u}{\partial s}$ is equal to $e^s \cos(t) \frac{\partial u}{\partial x} + e^s \sin(t) \frac{\partial u}{\partial y},$ what $\frac{\partial^2 u}{\partial d s^2}$ is equal to?
What expression are we suppose to get? I have been trying to figure out what to do for a hour, but I am quite lost. Doesn't the second partial derivative of $s$, give the same thing?
Any help would be appreciated. Thanks.