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f(x) = 2(cosx)^2 + sin(2x+π/2) - 1 = 1+cos2x + cos2x - 1 = 2cos2x
最小正周期 T = π
f(θ-π/8) = 2cos(2θ-π/4) = 1, cos(2θ-π/4) = 1/2,
0 < θ < π/2, -π/4 < 2θ-π/4 < 3π/4, 得 2θ-π/4 = π/3
2θ = π/4 + π/3，
cos2θ = cos(π/4)cos(π/3) - sin(π/4)sin(π/3)
= (√2/2)(1-√3)/2 = (√2-√6)/4
最小正周期 T = π
f(θ-π/8) = 2cos(2θ-π/4) = 1, cos(2θ-π/4) = 1/2,
0 < θ < π/2, -π/4 < 2θ-π/4 < 3π/4, 得 2θ-π/4 = π/3
2θ = π/4 + π/3，
cos2θ = cos(π/4)cos(π/3) - sin(π/4)sin(π/3)
= (√2/2)(1-√3)/2 = (√2-√6)/4
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