Given three coins of possibly different sizes which are arranged so that each is tangent to the other two, find the coin which is tangent to the other three coins. The solution is the inner Soddy circle, illustrated above.

Apollonius circle | Apollonius' problem | arbelos | bend | circumcircle | coin | Descartes circle theorem | Hart's theorem | Pappus chain | Soddy circles | sphere packing | Steiner chain | tangent circles