f(x)=cos(x/2)-sin^2(x/4)+3根号3/2sin(x/2)
=cos(x/2)-(1-cos(x/2))/2+3根号3/2sinx/2
=3/2cosx/2+3根号3/2sinx/2-1/2
=3sin(x/2+Pai/6)-1/2
2/3Pai=