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In geometry, the orthocentroidal circle of a non-equilateral triangle is the circle that has the triangle's orthocenter and
centroid In mathematics and physics, the centroid, also known as geometric center or center of figure, of a plane figure or solid figure is the arithmetic mean position of all the points in the surface of the figure. The same definition extends to any o ...
at opposite ends of its diameter. This diameter also contains the triangle's nine-point center and is a subset of the
Euler line In geometry, the Euler line, named after Leonhard Euler (), is a line determined from any triangle that is not equilateral. It is a central line of the triangle, and it passes through several important points determined from the triangle, includ ...
, which also contains the
circumcenter In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...
outside the orthocentroidal circle. Andrew Guinand showed in 1984 that the triangle's incenter must lie in the interior of the orthocentroidal circle, but not coinciding with the nine-point center; that is, it must fall in the open orthocentroidal disk punctured at the nine-point center... . The incenter could be any such point, depending on the specific triangle having that particular orthocentroidal disk. Furthermore, the Fermat point, the Gergonne point, and the symmedian point are in the open orthocentroidal disk punctured at its own center (and could be at any point therein), while the second Fermat point and Feuerbach point are in the exterior of the orthocentroidal circle. The set of potential locations of one or the other of the Brocard points is also the open orthocentroidal disk.. The square of the diameter of the orthocentroidal circle isAltshiller-Court, Nathan, ''College Geometry'', Dover Publications, 2007 (orig. Barnes & Noble 1952). D^2-\tfrac(a^2+b^2+c^2), where ''a, b,'' and ''c'' are the triangle's side lengths and ''D'' is the diameter of its
circumcircle In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...
.


References

{{commonscat, Orthocentroidal circle Circles defined for a triangle