Four Coins Problem
Given three coins of possibly different sizes which are arranged so that each is tangent to the other two, find thecoin which is tangent to the other three coins. The solution is the inner Soddy Circle.
See also Apollonius Circles, Apollonius' Problem, Arbelos, Bend (Curvature), Circumcircle,Coin, Descartes Circle Theorem, Hart's Theorem, Pappus Chain, Soddy Circles, SpherePacking, Steiner Chain
References
Oldknow, A. ``The Euler-Gergonne-Soddy Triangle of a Triangle.'' Amer. Math. Monthly 103, 319-329, 1996.
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