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Prove that cos 4x = 1 – 8 sin2 x cos2 x. |
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Answer» L.H.S. = cos 4x = cos 2(2x) (cos θ = 1 – 2 sin2 θ) = 1 – 2 sin2 2x = 1 – 2 (2 sin x cos x)2 [∵ sin 2x = 2 sin x cos x] = 1 – 2 x 4 sin2 x cos2 x = 1 – 8 sin2 x cos2 x = R.H.S. |
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