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Prove that r(1) r(2) + r(2) r(3) + r(3) ...

Prove that `r_(1) r_(2) + r_(2) r_(3) + r_(3) r_(1) = (1)/(4) (a + b + c)^(2)`

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We have already proved that `(1)/(r_(1)) + (1)/(r_(2)) + (1)/(r_(3)) = (1)/(r)`
`rArr r_(2) r_(3) + r_(3) r_(1) + r_(1) r_(2) = (r_(1) r_(2) r_(3))/(r) = (Delta^(3) s)/((s-a) (s-b) (s-c) Delta) = s^(2)`
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