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Prove that 2R cos A = 2R + r - r(1)...

Prove that `2R cos A = 2R + r - r_(1)`

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`2R + r - r_(1) = 2R + (s -a) tan.(A)/(2) - s tan.(A)/(2)`
`= 2R - a tan.(A)/(2)`
`= 2R - 2R sin A tan.(A)/(2)`
`=2R (1 - 2 sin^(2).(A)/(2))`
`= 2R cos A`
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