Cos 3A+Cos 5A+Cos 7A+Cos 9A=4Cos A Cos 2A Cos6A
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Step-by-step explanation:
(cos9A+cos3A)+(cos7A+cos5A)
=2cos6Acos3A+2cos6AcosA
=2cos6A(cos3A+cosA)
=2cos6A(2cos2AcosA)
=4cosAcos2Acos6A
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