f(x)=√3sinxcosx-cos2x+1/2=√3/2sin2x-cos2x+1/2
√[(√3/2)^2+1]=√7/2
令cosa=(√3/2)/(√7/2)=√(3/7)
sina=1/(√7/2)=2/√7
f(x)=(2/√7)sin(2x-a)+1/2
f(x)最小正周期T=2π/2=π
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