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Prove that a cos A + b cos B + c cos C =...

Prove that `a cos A + b cos B + c cos C = 4 R sin A sin B sin C.`

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`a cos A = 2R sin A cos A = R sin 2A`
`rArr a cos A + b cos B + c cos C`
`=R (sin 2A + sin 2B + sin 2C)`
`= 4R sin A sin B sin C`
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