Nikolaos Dergiades, Dao's theorem on six circumcenters associated with a cyclic hexagon,
Forum Geometricorum, 14 (2014) 243--246.

Abstract. We reformulate and give an elegant proof of a wonderful theorem of Dao Thanh Oai concerning the centers of the circumcircles of the six triangles each bounded by the lines containing three consecutive sides of the hexagon.

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