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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