if x=2 cos t - cos 2t, y=2 sin t – sin 2t find d2y/dx2 at t=pi/2

dx/dt=-2sint+2sin2t

1.  
   dx/dt=-2sint+2sin2t
   dy/dt=2cost-2cos2t
   dy/dx=(dy/dt)/(dx/dt)=(cost-cos2t)/(-sint+sin2t)
   dy/dx=2sin3t/2.sint/2/2cos3t/2sint/2
   =tan3t/2
   d²y/dx²=d/dx(dy/dx)
   =d/dt(dy/dx).dt/dx
   =(3/2).sec²3t/2.1/(-2sint+2sin2t)
   =3/4.1/cos²(3t/2).1/(sin2t-sint)
   =3/4.1/cos²(3t/2).1/2cos3t/2.sint/2
   =(3/8).1/(cos³3t/2.sint/2
   put t= pi/2
   d²y/dx²=3/8. 1/(cos³(3pi/4).sin(pi/4))
   =-3/2
   Thanks & Regards