ρ=aθ
所以x(θ)=ρcosθ=aθcosθ
y(θ)=ρsinθ=aθsinθ
根据曲率的参数表达式K=|x'y''-x''y'|/(x'^2+y'^2)^3/2
得K=1/[a(1+π)^3/2]
曲率半径R=1/K=a(1+π)^3/2