f(x)=向量a.向量b-2cos^2x.
=2sin（x+π/12)cos(x+π/12)+2sin(x-π/12)cos(x-π/12)-2cos^2x.
=sin(2x+π/6)+sin(2x-π/6)-2cos^2x.
=2sin[(2x+π/6+2x-π/6)/2]cos[(2x+π/6-2x+π/6)]/2-2cos^2x.
=2sin2xcosπ/6-(1+cos2x).
=2[(√3/2)sin2x-(1/2)cos2x-1.
∴f(x)=2sin(2x-π/6)-1.
1. f(x)的最小正周期T=2π/2=π.
2.y=g(x).
g(x)与fx)有什么关系?