1-cosα=1-[1-2sin(α/2)方]=2sin(α/2)方
1+cosα=1+[2cos(α/2)方-1]=2sin(α/2)方
(1-cosα)/(1+cosα)=2sin(α/2)方/2sin(α/2)方
(1-cosα)/(1+cosα)=tan(α/2)方