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Soddy's formula

If four circles A, B, C, and D, of radii r1, r2, r3, and r4, are drawn so that they do not overlap but each touches the other three, and if we let b1 = 1/r1, etc., then

(b1 + b2 + b3 + b4)2 = 2(b12 + b22 + b32 + b42).

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