有个公式：cos(a)cos(b)=1/2[cos(a+b)+cos(a-b)]
f(x)=2cos(x+π/4)cos(x-π/4)+√3sin2x
=cos(2x)+cos(π/2)+√3sin2x
=cos(2x)+√3sin2x
=2[cos(π/3)cos(2x)+sin(π/3)sin(2x)]
=2cos(2x-π/3)
值域：[-2,2]
最小正周期:π
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