>>686
(dx/dt)^2+(dy/dt)^2=(-2sint+2sin2t)^2+(2cost-2cos2t)^2
=8-8(sintsin2t+costcos2t)
=8-8cost
=16{sin(t/2)}^2

曲線の長さをLとすると
L=∫[0,π] √[16{sin(t/2)}^2] dt
=∫[0,π] 4sin(t/2) dt
=4[-2cos(t/2)][0,π]
=8