Triangle sides
Line segments perpendicular to the side midpoints (concur at the circumcenter)
Nine-point circle (centered at the nine-point center)
Note that the construction still works even if the orthocenter and circumcenter fall outside of the triangle.In geometry, the nine-point circle is a circle that can be constructed for any given triangle. It is so named because it passes through nine significant concyclic points defined from the triangle. These nine points are:
- The midpoint of each side of the triangle
- The foot of each altitude
- The midpoint of the line segment from each vertex of the triangle to the orthocenter (where the three altitudes meet; these line segments lie on their respective altitudes).[1][2]
The nine-point circle is also known as Feuerbach's circle (after Karl Wilhelm Feuerbach), Euler's circle (after Leonhard Euler), Terquem's circle (after Olry Terquem), the six-points circle, the twelve-points circle, the n-point circle, the medioscribed circle, the mid circle or the circum-midcircle. Its center is the nine-point center of the triangle.[3][4]
- ^ Altshiller-Court (1925, pp. 103–110)
- ^ Kay (1969, pp. 18, 245)
- ^ Kocik, Jerzy; Solecki, Andrzej (2009). "Disentangling a Triangle". Amer. Math. Monthly. 116 (3): 228–237. doi:10.4169/193009709x470065. Kocik and Solecki (sharers of a 2010 Lester R. Ford Award) give a proof of the Nine-Point Circle Theorem.
- ^ Casey, John (1886). Nine-Point Circle Theorem, in A Sequel to the First Six Books of Euclid (4th ed.). London: Longmans, Green, & Co. p. 58.