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Express 2cos5xcos2x as a sum containing only sines or cosines?

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please help!
asked Mar 6, 2013 in TRIGONOMETRY by anonymous Apprentice

1 Answer

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Sum of the cosine rule

Cos(A+B)+Cos(A-B) = 2CosACosB

Substitute 5x for A and 2x for B in above formulae


=cos7x+cos3x                          (Simplify)

Simplified expression is cos7x+cos3x


answered Mar 6, 2013 by bradely Mentor

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