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Prove thaacosA+bcosB+c cosClt=sdot...

Prove tha`acosA+bcosB+c cosClt=sdot`

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`a cos a + b cos B + c cos C`
`= R(2 sin A cos A + 2 sin B cos B + 2 sin C cos C)`
`=R (sin 2A + sin 2B + sin 2C)`
`=4R sin A sin B sin C`
`=(2)/(R) (2R^(2) sin A sin B sin C)`
`= (2)/(R) Delta`
`=2(rs)/(R) le s " " [:' R ge 2r]`
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