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Four Disks (Posted on 2005-02-02) Difficulty: 2 of 5
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.

See The Solution Submitted by David Shin    
Rating: 3.0000 (4 votes)

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Some Thoughts Added Observation Comment 5 of 5 |
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
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