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Prove that (r(1+r2))/1=2R...

Prove that `(r_(1+r_2))/1=2R`

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`r_(1) + r_(2) = 4R (sin.(A)/(2) cos.(B)/(2) cos.(C)/(2) + sin.(B)/(2) cos.(A)/(2) cos.(C)/(2))`
`= 4R cos.(C)/(2) (sin.(A)/(2) cos.(B)/(2) + sin.(B)/(2) cos.(A)/(2))`
`= 4R cos.(C)/(2) sin ((A + B)/(2))`
`= 4R (cos^(2).(C)/(2)) = 2R (1 + cos C)`
or `(r_(1) + r_(2))/(1 +cos C) = 2R`
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