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Prove that (r(1) -r)/(a) + (r(2) -r)/(b)...

Prove that `(r_(1) -r)/(a) + (r_(2) -r)/(b) = (c)/(r_(3))`

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`(r_(1) -r)/(a) + (r_(2) -r)/(b) = ((Delta)/(s-a) - (Delta)/(s))/(a) + ((Delta)/(s-b) -(Delta)/(s))/(b)`
`= (Delta (s-s +a))/(a(s-a)s) + (Delta(s-s + b))/(s(s -b)b)`
`= (Delta)/(s(s-a)) + (Delta)/(s(s -b))`
`= (Delta)/(s) ((s-b + s-a)/((s -a) (s-b)))`
`= (Delta)/(s) (c (s-c))/((s-a) (s-b) (s-c))`
`= (Delta c(s -c))/(Delta^(2))`
`= (c)/(r_(3))`
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