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Geometry Difficulty 5.3 AIME, harder Find the answer

Let ABCA B C be a triangle. The following diagram contains points P1,P2,,P7P_{1}, P_{2}, \ldots, P_{7}, which are the following triangle centers of triangle ABCA B C in some order: - the incenter II; - the circumcenter OO; - the orthocenter HH; - the symmedian point LL, which is the intersections of the reflections of BB-median and CC-median across angle bisectors of ABC\angle A B C and ACB\angle A C B, respectively; - the Gergonne point GG, which is the intersection of lines from BB and CC to the tangency points of the incircle with AC\overline{A C} and AB\overline{A B}, respectively; - the Nagel point NN, which is the intersection of line from BB to the tangency point between BB excircle and AC\overline{A C}, and line from CC to the tangency point between CC-excircle and AB\overline{A B}; and - the Kosnita point KK, which is the intersection of lines from BB and CC to the circumcenters of triangles AOCA O C and AOBA O B, respectively. Compute which triangle centers {I,O,H,L,G,N,K}\{I, O, H, L, G, N, K\} corresponds to PkP_{k} for k{1,2,3,4,5,6,7}k \in\{1,2,3,4,5,6,7\}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Let GG^{\prime} be the centroid of triangle ABCA B C. Recall the following. - Points O,G,HO, G^{\prime}, H lie on Euler's line of ABC\triangle A B C with OG:GH=1:2O G^{\prime}: G^{\prime} H=1: 2. - Points I,G,NI, G^{\prime}, N lie on Nagel's line of ABC\triangle A B C with IG:GN=1:2I G^{\prime}: G^{\prime} N=1: 2. Thus, OIHNO I \parallel H N with OI:HN=1:2O I: H N=1: 2. Therefore, we can detect parallel lines with ratio 2:12: 1 in the figure. The only possible pairs are P2P4P7P5P_{2} P_{4} \parallel P_{7} P_{5}. Therefore, there are two possibilities: (P2,P7)\left(P_{2}, P_{7}\right) and (P4,P5)\left(P_{4}, P_{5}\right) must be (O,H)(O, H) and (I,N)(I, N) in some order. Intuitively, HH should be further out, so it's not unreasonable to guess that P2=O,P7=H,P4=IP_{2}=O, P_{7}=H, P_{4}=I, and P5=NP_{5}=N. 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 GG and NN both lie on OIO I (they are insimilicenter and exsimilicenter of incircle and circumcircle, respectively). Thus, H,G,N,IH, G, N, I lie on isogonal conjugate of OIO I, known as the Feuerbach's Hyperbola. It's also known that OIO I 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 H,G,N,IH, G, N, I and is tangent to OIO I. It's then not hard to see that P6=GP_{6}=G. 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, H,L,K,OH, L, K, O lies on isogonal conjugate of OHO H, which is the Jerabek's Hyperbola. One can see that H,L,K,OH, L, K, O lies on the same branch. Moreover, they lie on this hyperbola in this order because the isogonal conjugates (in order) are OO, centroid, nine-point center, and HH, which lies on OHO H in this order. Using this fact, we can identity P5=LP_{5}=L and P1=KP_{1}=K, completing the identification.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.