Prove that:

cos 3A + cos 5A + cos 7A + cos 15 A = 4 cos 4 A cos 5 A cos 6 A

Formula: - (i)

Taking L.H.S

Cos 3 A + cos 15 A + cos 5 A + cos 7 A

Applying The above formula we get,

⇒ cos3A + cos15A + cos5A + cos7A = 2cos9Acos6A + 2cos6AcosA

⇒ cos3A + cos15A + cos5A + cos7A = 2cos6A(cos9A + cosA)

Again applying the formula we get,

⇒ cos3A + cos15A + cos5A + cos7A = 2cos6A(2cos4Acos5A)

L.H.S = R.H.S

Hence, Proved.