cos^2(x+π/3)+cos^2(x-π/3)
=(cosx/2-根号3*sinx/2)^2+(cosx/2+根号3*sinx/2)^2
=(cosx)^2/2+3(sinx)^2/2
=1/2+(sinx)^2
f(x)=cos^2x+cos^2(x+π/3)+cos^2(x-π/3)
=(cosx)^2+1/2+(sinx)^2
=3/2,
证明:函数f(x)=cos^2+cos^2(x+π/3)+cos^2(x-π/3)是常数函数
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