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The ex-radii r1,r2,r3 or Delta ABC are ...

The ex-radii `r_1,r_2,r_3` or `Delta ABC` are in H.P. Show that its sides a,b,c are in A.P.

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`r_1=D/(s-a),r_2=D/(s-b),r_3=D/(s-c)`
`D/(s-a),D/(s-b),D/(s-c)` are in HP
s-a,s-b,s-c are in Ap
a,b,c are in AP.
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