∵cos^6x周期为π
∫[0,2π] cos^6x dx = 2∫[0,π] cos^6x dx = 2I
令x=π/2-y => y=π/2-x,dy=-dx
当x=0,y=π/2 // 当x=π,y=-π/2
I = -∫[π/2,-π/2] cos^6(π/2-y) dy
= ∫[-π/2,π/2] sin^6y dy
= 2∫[0,π/2] sin^6y dy <=sin^6y是偶函数
= 2 * (6-1)!!/(6!!) * π/2
= 2*(5!!)/(6!!) * π/2
= 5π/16
∴原式= 2I = 2*5π/16
= 5π/8
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