f(x) = a.b-1
=（2sinx,cosx）.（√3cosx,2cosx） -1
= 2sinxcosx + 2(cosx)^2 -1
= sin2x+cos2x
= sin(π/2-2x)+ cos(π/2-2x)
= sin2(π/4-x) + cos2(π/4-x)
f(x) = f(π/4-x)
f(π/8-x) = f(π/4-(π/8-x)
=f(π/8+x)
对称轴:y =π/8