thx for your advice. Ok, this is my last question. I want you to show me in detail how to get d^2u/dx^2 . please dont't blame me whether i didn't try it myself because actually I did but I stuck with this kind of problems for 3 days .(I have the limit of time before my test!) thanks a lot
I've tried to apply the chain rule again as you have told me to do so . I got these following results.
as from du/dx = {(1/3)[x^(-2/3)]}du/dz
thus du/dz = [3x^(-2/3)]du/dx
and then [d(du/dz)]/dx = d[(3x^2/3)(du/dx)]/dx
using product rule; [d(du/dz)]/dx = [2x^(-1/3)]du/dx +...
can you show me why the term \frac{1}{3}x^{-\frac{2}{3}}\frac{d^{2}u}{dz^{2}} is wrong?
(I have already followed and checked the chain rule step by step but I still got this term.)