dx/dt=-sint
d^2x/d^2t=-cost
dy/dt=4*sin^3(t/2)*cos(t/2)*1/2=2sin^3(t/2)*cos(t/2)=sint*sin^2(t/2)
d^2y/d^2t=cost*sin^2(t/2)+sint*2sin(t/2)*cos(t/2)*1/2=
=cost*sin^2(t/2)+sin^2t/2
(d^2y/d^2t):(d^2x/d^2t)=d^2y/d^2x=-sin^2(t/2)-tgt*sint
