f(x)=a*b+1
=2cosxcosx+2cosxsin(π-x)+1
=2cos^2x+2sinxcosx+1
=cos2x+1+sin2x+1
=cos2x+sin2x+2
=√2sin(2x+π/4)+2
所以：最小正周期等于2π/2=π.
因为：
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