Let be a triangle. The following diagram contains points , which are the following triangle centers of triangle in some order: - the incenter ; - the circumcenter ; - the orthocenter ; - the symmedian point , which is the intersections of the reflections of -median and -median across angle bisectors of and , respectively; - the Gergonne point , which is the intersection of lines from and to the tangency points of the incircle with and , respectively; - the Nagel point , which is the intersection of line from to the tangency point between excircle and , and line from to the tangency point between -excircle and ; and - the Kosnita point , which is the intersection of lines from and to the circumcenters of triangles and , respectively. Compute which triangle centers corresponds to for .
Solution
Let be the centroid of triangle . Recall the following. - Points lie on Euler's line of with . - Points lie on Nagel's line of with . Thus, with . Therefore, we can detect parallel lines with ratio in the figure. The only possible pairs are . Therefore, there are two possibilities: and must be and in some order. Intuitively, should be further out, so it's not unreasonable to guess that , and . Alternatively, perform the algorithm below with the other case to see if it fails. To identify the remaining points, we recall that the isogonal conjugate of and both lie on (they are insimilicenter and exsimilicenter of incircle and circumcircle, respectively). Thus, lie on isogonal conjugate of , known as the Feuerbach's Hyperbola. It's also known that is tangent to this line, and this hyperbola have perpendicular asymptotes. Using all information in the above paragraph, we can eyeball a rectangular hyperbola passing through and is tangent to . It's then not hard to see that . Finally, we need to distinguish between symmedian and Kosnita points. To do that, recall that Kosnita point is isogonal conjugate of the nine-point center (not hard to show). Thus, lies on isogonal conjugate of , which is the Jerabek's Hyperbola. One can see that lies on the same branch. Moreover, they lie on this hyperbola in this order because the isogonal conjugates (in order) are , centroid, nine-point center, and , which lies on in this order. Using this fact, we can identity and , completing the identification.