(please see image link below) Currently stuck on this problem involving the chain rule and partial derivatives. Having looked at the highlighted part of the solution, I can't figure out two things:
- Surely the $\sin\psi$ term should be zero, as it's just taking the partial derivative with respect to $r$ of $\sin\psi$?
- Likewise, shouldn't the $r \cos\psi$ term be simply $\cos\psi$, as it's just the partial derivative of $\sin\psi$ with respect to $\psi$?
Any advice would be much appreciated.