dl²=dx²+dy²
x=pcosθ=2θsinθ
y=psinθ=2θcosθ
dx=2(sinθ+θcosθ)dθ
dy=2(cosθ-θsinθ)dθ
dl=2(1+θ²)^(1/2)dθ
总长度l=∫dl=...具体怎么积我有点忘了.