因为5π/2<θ<3π,所以5π/4<θ/2<3π/2,所以sinθ/2<0,cosθ/2<0,tanθ/2 >0
因为cosθ=1-2(sinθ/2)^2=2(cosθ/2)^2,所以sinθ/2=-根号((1-cosθ)/2)
cosθ/2=-根号((1+cosθ)/2),tanθ/2=(sinθ/2)/(cosθ/2)=根号((1-cosθ)/(1+cosθ))