dx/dt=a(1-cost),dy/dt=asint
由公式：
弧长S=∫√[（dx/dt）^2+(dy/dt)^2] dt 积分从0到2π
=∫√a^2[1-2cost+(cost)^2]+(asint)^2] dt
=a∫√(2-2cost) dt
=a∫2|sin(t/2)| dt
=8πa