Four disks are arranged in a plane such that each is externally tangent to two others. Prove that the four points of tangency lie on a circle.

Nice solution Jer. Using your notation, let E be the intersection of diagonals AC and BD and P, Q, R, and S the incenters of triangles EAB, EBC, ECD, and EDA respectively. Prove that the incenters also lie on a circle.

Posted by Bractals on 2005-02-04 23:03:42