Maths Olympiad Prep

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Geometry Difficulty 8.4 Shortlist Prove it United States

Let ABCABC be an acute triangle. Circle ω1\omega_1, with diameter ACAC, intersects side BCBC at FF (other than CC). Circle ω2\omega_2, with diameter BCBC, intersects side ACAC at EE (other than CC). Ray AFAF intersects ω2\omega_2 at KK and MM with AK<AMAK < AM. Ray BEBE intersects ω1\omega_1 at LL and NN with BL<BNBL < BN. Prove that lines ABAB, MLML, NKNK are concurrent.

(This problem was suggested by Steve Dinh.)

Solution

Let DD be the foot of the perpendicular from CC to ABAB and HH be the orthocenter of ABC\triangle ABC. Note first that ω1\omega_1 and ω2\omega_2 both intersect ABAB at DD. By Power of a Point, LHHN=CHHD=KHHMLH \cdot HN = CH \cdot HD = KH \cdot HM, implying that KLMNKLMN is a cyclic quadrilateral. Noting that ACAC and BCBC are perpendicular bisectors of the diagonals of KLMNKLMN, we conclude that the center of its circumcircle is CC. Observe now that since ANC=ALC=90\angle ANC = \angle ALC = 90^\circ, we have that ANAN and ALAL are tangent to the circumcircle of KLMNKLMN. Thus, HH is on the polar of AA. Similarly, HH is on the polar of BB, and so by Brocard's theorem, KNKN and LMLM meet on the polar of HH, which is ABAB.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.