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The Circles on the Diagonals of a Complete Quadrilateral as Diameters are Coaxal. Furthermore, the Orthocenters of the four Triangles of a Complete Quadrilateral are Collinear on the Radical Axis of the Coaxal Circles.
See also Coaxal Circles, Collinear, Complete Quadrilateral, Diagonal (Polygon), Orthocenter, Radical Axis
References
Johnson, R. A.  Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle.  Boston, MA:
  Houghton Mifflin, p. 172, 1929.