tan(x/2)=sin(x/2)/cos(x/2)=2sin(x/2)cos(x/2)/2cos²(x/2)=sinx/(1+cosx)=sinx(1-cosx)/(1+cosx)(1-cosx)=sinx(1-cosx)/(1-cos²x)=sinx(1-cosx)/sin²x=(1-cosx)/sinx