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

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

1 Answer

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2cos5xcos2x

Sum of the cosine rule

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

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

=cos(5x+2x)+cos(5x-2x)

=cos7x+cos3x                          (Simplify)

Simplified expression is cos7x+cos3x

 

answered Mar 6, 2013 by bradely Mentor

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