题目

已知函数f(x)=cosx·cos(x-θ)-1/2cosθ,其中x∈R,0

提问时间：2021-03-04

答案

(1)
cos(2x-θ)=cos[x+(x-θ)]=cosxcos(x-θ)-sinxsin(x-θ)
cosθ = cos[x-(x-θ)]=cosxcos(x-θ)+sinxsin(x-θ)
两式相加得：cos(2x-θ)+cosθ=2cosxcos(x-θ)
cosxcos(x-θ)=(1/2)[cos(2x-θ)+cosθ]
f(x)=(1/2)[cos(2x-θ)+cosθ] - (1/2)cosθ=(1/2)cos(2x-θ)
f(x)(MAX)=1/2
f(π/3)=(1/2)[cos(2π/3-θ)=1/2
所以θ=2π/3
(2)
g(x)=2*[(1/2)cos(3x-2π/3)]=cos(3x-2π/3)
∵0≤x≤π/3
∴-2π/3≤3x-2π/3≤π/3
∴g(x)的最小值为cos(-2π/3) = - 1/2