=[2sinθ/2cosθ/2+2(cosθ/2)^2]*(sinθ/2-cosθ/2)/√[4(cosθ/2)^2]
=2cosθ/2*(sinθ/2+cosθ/2)(sinθ/2-cosθ/2)/2|cosθ/2|
=cosθ/2*(-cosθ)/|cosθ/2|
∵(270＜θ＜360,135＜θ/2＜180 )
∴=cosθ