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Prove that (b+c)cosA+(c+a)cosB+(a+b)cosC...

Prove that `(b+c)cosA+(c+a)cosB+(a+b)cosC=2sdot`

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`(b + c) cos A + (c + a) cos B + (a + b) cos C`
`= (b cos A + a cos B) + (c cos A + a cos C) + (b cos C + c cos B) = c + b + a = 2s`
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