Diameter of inscribed circle

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incribed circle

How can i express diameter of inscribed circle in terms of radius of three circles.

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$a=\frac{1}{R_1},\, b=\frac{1}{R_2},\, c=\frac{1}{R_3},\, x=\frac{1}{R},\, $ then

$x = a+b+c+2\sqrt{ab+bc+ac}$

I don't know derivation of this relationship. Discovered him famous British chemist F. Soddy.

This relationship is known from Czech books on recreational mathematics from 1967.