2sinθ=1+cosθ
2*2sin(θ/2)cos(θ/2)=1+2cos²(θ/2)-1
2sin(θ/2)cos(θ/2)=cos²(θ/2)
cos(θ/2)=0时,tan(θ/2)不存在
cos(θ/2)不等于0,则两边除以cos²(θ/2),
2sin (θ/2)/cos(θ/2)=1
tan(θ/2)=sin(θ/2)/cos(θ/2)=1/2