tanθ=tan(2*θ/2)=2tan(θ/2)/(1-tan(θ/2)^2)
2/tanθ=(1-tan(θ/2)^2)/tan(θ/2)=1/tan(θ/2)-tan(θ/2)
所以：
tan(θ/2)-1/tan(θ/2)=-2/tanθ,得证.