Maths Olympiad Prep

Library / /33 of 397

Geometry Difficulty 4.8 AIME Prove it Taiwan

Let the incircle of acute triangle ABCABC be ω\omega, the circumcircle be Ω\Omega, and let the midpoint of side BCBC be MM. Let the incircle ω\omega be tangent to CACA, ABAB at points EE, FF respectively, and let line EFEF meet the circumcircle Ω\Omega at points PP, QQ. Take a point RR on the circumcircle Γ\Gamma of MPQ\triangle MPQ such that MRMR is perpendicular to EFEF. Prove that line ARAR, circle Γ\Gamma, and circle ω\omega meet at a single point.

Solution

Let EFEF meet BCBC at point SS, and let ω\omega be tangent to BCBC at point DD. Because (B,C;D,S)=1(B, C; D, S) = -1, we have SDSM=SBSC=SPSQSD \cdot SM = SB \cdot SC = SP \cdot SQ, that is, D,M,P,QD, M, P, Q are concyclic. Let (AEF)\odot(AEF) meet Ω\Omega again at point TT, and let AIAI meet Ω\Omega again at point NN. Then, since TBFTCE\triangle TBF \sim \triangle TCE, we know TBTC=BFCE=BDCE\frac{TB}{TC} = \frac{BF}{CE} = \frac{BD}{CE}, that is, T,D,NT, D, N are collinear.

Figure 1

Suppose Γ\Gamma meets ω\omega again at point KK, and let the radical axis of (AEF)\odot(AEF) and Γ\Gamma be \ell. Considering (AEF)\odot(AEF), Γ\Gamma, ω\omega, by the radical center theorem we know ,DK,EF\ell, DK, EF are concurrent; considering next (AEF),Γ,Ω\odot(AEF), \Gamma, \Omega, by the radical center theorem we know ,PQ,AZ\ell, PQ, AZ are concurrent, so ,DK,EF,AZ\ell, DK, EF, AZ are concurrent at point XX, and A,T,K,DA, T, K, D are concyclic.

Therefore
AKD=ATD=ATN=(AN,BC)=RMD=RKD \angle AKD = \angle ATD = \angle ATN = \angle(AN, BC) = \angle RMD = \angle RKD
that is, R,A,KR, A, K are collinear, which completes the proof.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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