因为 [sin(x/2)+cos(x/2)]²＝1+2sin(x/2)cos(x/2)=1+sinx[sin(x/2)-cos(x/2)]²＝1-2sin(x/2)cos(x/2)=1-sinx1+cosx=2cos²(x/2),且当 x/2∈（0,π/4）时,有sin(x/2)