cos5x . cos4x = cos3x . cos2x
⇔ 1/2.[cos(5x-4x)+cos(5x+4x)] = 1/2.[cos(3x-2x)+cos(3x+2x)]

⇔ 1/2.[cosx + cos9x]                 = 1/2.[cosx + cos5x]

⇔ 1/2.cosx + 1/2.cos9x             = 1/2.cosx + 1/2.cos5x

 đơn giản 1/2

⇔ cosx + cos9x = cosx + cos5x

⇔ cosx + cos9x - cosx - cos5x=0

⇔ cos9x - cos5x = 0

⇔ -2.sin7x.sin2x = 0

⇔ sin7x.sin2x = 0

⇔ [sin7x=0sin2x=0[sin7x=0sin2x=0 

⇔ [x=k.π/7x=k.π/2[x=k.π/7x=k.π/2  (k∈Z)
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