Answer :
Take L.H.S.
cos 3A + cos 5A + cos 7A + cos 15A
= (cos 5A + cos 3A) + (cos 15A + cos 7A)
= 2 cos 4A cos A + 2 cos 11A cos 4A
= 2 cos 4A (cos 11A + cos A)
= 4 cos 4A cos 5A cos 6A
= R.H.S.
Hence Proved
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