1.

Prove that  cos 4x = 1 – 8 sin2 x cos2 x.

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)[∵ 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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