Maths Olympiad Prep

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, 2022

Geometry Difficulty 5.1 AIME, harder Prove it Taiwan

平面上 ABCABC 為銳角三角形,其外心為 OO,外接圓為 Ω\Omega。分別在線段 AB,ACAB, AC 上各取一點 D,ED, E,並作過 AADEDE 垂直的直線 \ell。設 \ell 分別與三角形 ADEADE 的外接圓及 Ω\Omega 再交於點 P,QP, Q。令直線 OQOQBCBC 交於點 NN,直線 OPOPDEDE 交於點 SS,且點 WW 為三角形 AOSAOS 的垂心。
試證:S,N,O,WS, N, O, W 四點共圓。

Solution

Let D,ED', E' be points on lines AB,ACAB, AC, respectively, such that DED'E' is parallel to DEDE, and passes through NN. From
NDB=90BAQ=OQB=NQB, \angle ND'B = 90^\circ - \angle BAQ = \angle OQB = \angle NQB,
we see that B,D,N,QB, D', N, Q are concyclic. Since
QDE=QBC=QAE, \angle QD'E' = \angle QBC = \angle QAE',
A,D,Q,EA, D', Q, E' are concyclic. Therefore, ADEP+ADEQ\triangle ADE \cup P \stackrel{+}{\sim} \triangle AD'E' \cup Q.
Let S,TAS', T \neq A be the intersections of ASAS and DED'E', Ω\Omega, respectively. Then
QTS=QTA=(QO,AQ)=QNS \angle QTS' = \angle QTA = \angle(QO, \perp AQ) = \angle QNS'
gives the concyclicity of Q,T,N,SQ, T, N, S'. From the similarity ADE{P,S}+ADE{Q,S}\triangle ADE \cup \{P, S\} \stackrel{+}{\sim} \triangle AD'E' \cup \{Q, S'\}, we see that PSPS is parallel to QSQS'. According to Reim's theorem, S,N,O,TS, N, O, T are concyclic. Thus,
OWS=SAO=OTA=ONS, \angle OWS = \angle SAO = \angle OTA = \angle ONS,
as desired.

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