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Express cos7xcos2x in the form of A(cosBx+cosCx) where A,B and C are constants to be determined.
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Express cos7xcos2x in the form of A(cosBx+cosCx) where A,B and C are constants to be determined.

User Martita
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\cos(7x+2x)=\cos7x\cos2x-\sin7x\sin2x

\cos(7x-2x)=\cos7x\cos2x+\sin7x\sin2x


\implies\cos9x+\cos5x=2\cos7x\cos2x

\implies\cos7x\cos2x=\frac12(\cos9x+\cos5x)

\implies A=\frac12,B=9,C=5
User Ragesh Chakkadath
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