两倍角公式:
cosθ=1-2sin²(θ/2)
sinθ=2sin(θ/2)cos(θ/2)
(1-cosθ)/sinθ
=2sin²(θ/2)/[2sin(θ/2)cos(θ/2)]
=sin(θ/2)/cos(θ/2)
=tan(θ/2)