(1)
f(x)=1/2sin2xsinψ+cos²xcosψ-1/2sin(π/2+ψ)
=1/2*sin2x*sinψ+cos²xcosψ-1/2*cosψ
=1/2*(sin2x*sinψ+cos2x*cosψ)=1/2*cos(2x-ψ)
由 f(π/6)=1/2*cos(2*π/6-ψ)=1/2
得 2*π/6-ψ=0 ==> ψ=π/3
(2)
令 x=2t
则 y=g(t)=1/2*cos(4t-π/3)
若 0 ≤ t ≤ π/4
则 -π/3 ≤ 4t-π/3 ≤ 2π/3
因为cos在区间 [-π/3 ,0]是增函数,在 [0,2π/3]上是减函数,所以
y=1/2*cos(4t-π/3) 在[0,π/4]的最大值 fmax=1/2*cos(0)=1/2
最小值 fmin=1/2*cos(2π/3)=-1/2*1/2= -1/4