Maths Olympiad Prep

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Geometry Difficulty 6.9 National olympiad Prove it

Let ABCA B C be an acute-angled triangle with circumcircle ω\omega. Prove that there exists a point JJ with the following property: If XX is an interior point of ABCA B C, the rays AXA X, BXB X and CXC X intersect the circle ww again at points A1,B1A_{1}, B_{1} and C1C_{1}, and the points A2A_{2}, B2B_{2} and C2C_{2} are symmetric to A1,B1A_{1}, B_{1} and C1C_{1} with respect to the midpoints of the segments BC,CA\overline{B C}, \overline{C^{\prime} A} and AB\overline{A B}, respectively, then the four points A2,B2,C2A_{2}, B_{2}, C_{2} and JJ lie on a common circle.

Solution

We show that the point JJ, usually denoted as HH, the orthocenter of triangle ABCABC, has the described property. For this, let aa be the line through AA parallel to BCBC, and the lines bb and cc are defined analogously. No two of the three lines a,ba, b, and cc are parallel, and therefore, the intersection point AA' of bb with cc and the two analogously defined points BB' and CC' exist.
The quadrilateral ACJBA' C J B has right angles at BB and CC and is therefore a cyclic quadrilateral. Since the reflection at the midpoint of the segment BC\overline{BC} maps point AA to AA' and swaps BB with CC, it transforms ω\omega into the circumcircle of the just found cyclic quadrilateral. Consequently, A2A_2 lies on this circle, and ΛA2\Lambda' A_2 is parallel to AXAX.
Let the centroid of triangle ABCABC be SS. The central dilation σ\sigma with center SS and factor 2-2 maps AA to AA'; the image of XX is denoted by XX'. Then, the lines AXAX and AXA'X'' are parallel, and therefore, XX' lies on the line AA2A'A_2. Similar arguments can be made by replacing AA with BB and CC, and we learn in total: The three lines AA2,BB2A'A_2, B'B_2, and CC2C''C_2 intersect at XX'.
!

If X=JX' = J, the points A2,B2A_2, B_2, and C2C_2 coincide with JJ, and the claim is trivial. From now on, let XJX' \neq J.
Since \varangleACJ=90\varangle A' C J = 90^\circ, the segment AJ\overline{A' J} is, by Thales' theorem, a diameter of the circumcircle of the quadrilateral ACJBA' C J B. Again, by Thales' theorem, \varangleJA2A=90\varangle J A_2 A' = 90^\circ and therefore also \varangleJA2X=90\varangle J A_2 X' = 90^\circ. Thus, the point A2A_2 lies, once again by Thales' theorem, on the circle with diameter JX1J X^{-1}. For similar reasons, the points B2B_2 and C2C_2 also lie on this circle, and in particular, we have now found a circle on which all four points A2,B2,C2A_2, B_2, C_2, and JJ lie.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.