1. The problem statement, all variables and given/known data If u=f(x,y) where x=escost and y=essint show that d2u/dx2+d2u/dy2 = e-2s[d2u/ds2+d2u/dt2 2. Relevant equations http://s11.postimage.org/sjwt1wkvl/Untitled.jpg 3. The attempt at a solution ok i don't understand how they got to that i don't know what d/ds is supposed to do exactly, i thought it was the second partial derivative of s, but it seems that it's not if we check the answer. i basically got e^s cost (d^2/dx^2) + e^s sint (d^2u/dy^2), but they got 6 terms. i am kinda lost, can you explain to me what d/ds is supposed to do?