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.
http://forumgeom.fau.edu/FG2014volume14/FG201424.pdf
http://forumgeom.fau.edu/FG2014volume14/FG201429.pdf
