In Euclidean geometry, Kosnita's theorem is a property of certain circles associated with an arbitrary triangle.

Let ${\displaystyle ABC}$ be an arbitrary triangle, ${\displaystyle O}$ its circumcenter and ${\displaystyle O_{a},O_{b},O_{c}}$ are the circumcenters of three triangles ${\displaystyle OBC}$, ${\displaystyle OCA}$, and ${\displaystyle OAB}$ respectively. The theorem claims that the three straight lines ${\displaystyle AO_{a}}$, ${\displaystyle BO_{b}}$, and ${\displaystyle CO_{c}}$ are concurrent. ^[1] This result was established by the Romanian mathematician Cezar Coşniţă (1910-1962). ^[2]

Their point of concurrence is known as the triangle's Kosnita point (named by Rigby in 1997). It is the isogonal conjugate of the nine-point center. ^{[3] [4]} It is triangle center ${\displaystyle X(54)}$ in Clark Kimberling's list. ^[5] This theorem is a special case of Dao's theorem on six circumcenters associated with a cyclic hexagon in. ^{[6] [7] [8] [9] [10] [11] [12]}

References

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