y=2sin(x/2)cos(x/2)/[1+2cos²(x/2)-1]
=2sin(x/2)cos(x/2)/[2cos²(x/2)]
=sin(x/2)/cos(x/2)
=tan(x/2)
所以T=π/(1//2=2π请问这一步怎么来的？y=2sin(x/2)cos(x/2)/[1+2cos²(x/2)-1]倍角公式