1.

Prove that sin 4A = 4sinA cos3A – 4 cosA sin3A.

Answer»

L.H.S. = sin 4A

= 2 sin 2A- cos 2A 

= 2(2 sin A cosA)(cos2 A – sin2 A)

= 4 sin A • cos3 A – 4 cos A sin3 A = R.H.S.



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