sinx=2sin(x/2)cos(x/2)
1=sin^2(x/2)+cos^2(x/2)
1+sinx=sin^2(x/2)+cos^2(x/2)+2sin(x/2)cos(x/2)
=(sin(x/2)+cos(x/2))^2
同理：
1-sinx=sin^2(x/2)+cos^2(x/2)-2sin(x/2)cos(x/2)
=(sin(x/2)-cos(x/2))^2