IT IS A GOOD QUESTION!!!
It's a question about coords, combination of derivatives, and vector calculus altogether.
It's about expressing the original combination of derivatives in another co-ords, usually orthogonal curvy ones. For example, polar co-ords are curvy, and dr and dθ at any given point are orthogonal to each other.
It's usually explained in a vector calculus book or a calculus book for physicians. Untill now, I haven't got a satisfactory explaination for this kind of transfer in any books.
However, I do find a particular solution for your coords and your case.
According to Chain Rule
Thus you can compute the right of your equation and use cos²θ+sin²θ=1 to prove it equates the left.
Last edited by George,Y (2006-05-13 21:39:22)