f(sin(x/2))=cosx+1
=1-2(sin(x/2))^2+1
=2-2(sin(x/2))^2
令y=sin(x/2)
则f(y)=2-2y^2
令y=cos(x/2)
f(cos(x/2))=2-2(cos(x/2))^2
=2(sin(x/2))^2