θ∈(5π/2,3π),第二象限
sinθ=24/25,cosθ=-7/25
θ∈(5π/2,3π),θ/2∈(5π/4,3π/2),第四象限 cos(θ/2)>0
cosθ=2cos^2(θ/2)-1
2cos^2(θ/2)=18/25
cos^2(θ/2)=9/25 cos(θ/2)=3/5