dy/dθ=Rcos(α/2)((2π/α)tan(α/2-θ)cos(2πθ/α)-sin(2πθ/α)/cos^2(α/2-θ))=0
(2π/α)sin(α/2-θ)cos(α/2-θ)cos(2πθ/α)=sin(2πθ/α)
(2π/α)sin(α/2-θ)cos(α/2-θ)=tan(2πθ/α)
うーん