∵tanθ=2tan(θ/2)/[1-tan²(θ/2)]
∴1-tan²(θ/2)=2tan(θ/2)/tanθ
∴原式=[tan²(θ/2)－1]/tan(θ/2)
=-2tan(θ/2)/tanθtan(θ/2)
=-2/tanθ
=-2cotθ