1. The problem statement, all variables and given/known data z=f(x,y) x=escos(t) y=essin(t) show d2z/dx2+d2z/dy2 = e-2s[d2z/ds2+ d2/dt2 2. Relevant equations dz/dt=dz/dz(dx/dt)+(dz/dy)dy/dr The product rule 3. The attempt at a solution I found d2x/dt2=2e2ssin(t)cos(t)d2z/dydx + e2scos2(t)dz/dy2 But, now I'm lost. It doesn't seem to be going anywhere. I don't know where I am going to get rid of the d2z/dydx term. Thank your for your help.