dy/dx=(dy/dt) / (dx/dt)=-sint / 2t
d²y/d x²=d(dy/dx)/dx=[d(dy/dx)/dt] / (dx/dt)
=d(-sint / 2t)/dt / 2t
=[ -1/2 *( -2tcost+2sint)/t^2]/ 2t
=(tcost-sint) / 2t^3