应用公式：
sina=2sina/2*cosa/2
cosa=2(cosa/2)^2-1=1-2(sina/2)^2
（1+sinθ-cosθ0/(1+sinθ+cosθ）
=(1+2sinθ/2*cosθ/2-1+2sin^2θ/2)/(1+2sinθ/2*cosθ/2+2cos^2θ/2-1)
=(2sinθ/2*cosθ/2+2sin^2θ/2)/(2sinθ/2*cosθ/2+2cos^2θ/2)
=2sinθ/2(sinθ/2+cosθ/2)/2cosθ/2(sinθ/2+cosθ/2)
=sinθ/2/cosθ/2
=tanθ/2