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Geometry Difficulty 5.1 AIME, harder Prove it Taiwan

Let the circumcircle of triangle ABCABC be Ω\Omega, the incenter be II, and the AA-excenter be JJ. Let TT be the reflection of JJ over BCBC, and let PP be the intersection of BCBC and ATAT. If the circumcircle of AIP\triangle AIP meets BCBC at XPX \neq P, and YAY \neq A lies on Ω\Omega such that IA=YI\overline{IA} = \overline{YI}, prove that: the circumcircle of IXY\triangle IXY is tangent to line AIAI.

Solution

(∠ denotes directed angles.)

Figure 1

Let MM, NN be the midpoints of BC\overline{BC}, IJ\overline{IJ} respectively, let NN' be the reflection of NN over MM, and let DD be the foot of the perpendicular from JJ to BCBC. It is well known that IMADIM \parallel AD, and also MNBCDJMN \perp BC \perp DJ, so ADJ\triangle ADJ and IMN\triangle IMN are homothetic, hence since TT is the reflection of JJ over DD, we know ATINAT \parallel IN'. Let OO be the circumcenter of ABC\triangle ABC; from YON=2YAN=YIN\angle YON = 2\angle YAN = \angle YIN we obtain that Y,I,N,OY,I,N,O are concyclic. From CNN=NNC=ONC=NCO\angle CN'N = \angle N'NC = \angle ONC = \angle NCO and the Shooting Lemma (雞爪定理), we know NONN=NC2=NI2NO \cdot NN' = \overline{NC}^2 = \overline{NI}^2, that is, AIAI is tangent to (ION)\odot(ION'). Let QQ be the intersection of NYNY and BCBC; from QCN=BAN=NAC=NYC\angle QCN = \angle BAN = \angle NAC = \angle NYC and the Shooting Lemma, we know NQNY=NC2=NI2NQ \cdot NY = \overline{NC}^2 = \overline{NI}^2, so (IQY)\odot(IQY) is tangent to AIAI. From QIN=NYI=NOI=NIN\angle QIN = \angle NYI = \angle NOI = \angle N'IN we obtain that Q,I,NQ,I,N' are collinear. Also IXQ=IAP=NIN=ION=IYQ\angle IXQ = \angle IAP = \angle NIN' = \angle ION = \angle IYQ, so X(IQY)X \in \odot(IQY), hence (IXY)\odot(IXY) is tangent to AIAI.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.