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cos4x=1-8sin^2xcos^2x...

`cos4x=1-8sin^2xcos^2x`

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`LHS:`

`cos 4 x=cos 2(2 x) `

`=1-2 sin ^{2} 2 x`

`=1-2(sin 2 x)^{2} `

`=1-2(2 sin x cos x)^{2} `

`=1-8 sin ^{2} x cos ^{2} x `

`=text { RHS }`

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