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Prove that cosA+cosB+cosC=1+r/R...

Prove that `cosA+cosB+cosC=1+r/R`

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`cosA + cos B + cosC = 1 + 4 sin((A)/(2)) sin((B)/(2)) sin((C)/(2))`
`= 1 + ([4R sin (A//2) sin (B//2) sin(C//2)])/(R) = 1 + (r)/(R)`
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