Triangle $ABC$ has circumcenter $O$. $D$ is an arbitrary point on $BC$, and $AD$ intersects $\odot(ABC)$ at $E$. $S$ is a point on $\odot(ABC)$ such that $D, O, E, S$ are colinear. $AS$ intersects $BC$ at $P$. $Q$ is a point on $BC$ such that $D, O, A, Q$ are concylic. Prove that $\odot(ABC)$ is tangent to $\odot (APQ)$. Proposed by chengbilly
Problem
Source: 2024 imocsl G2 (independent quiz P2)
Tags: geometry, IMOC
10.08.2024 19:39
let $AQ$ intersect $(ABC)$ again at $F$ claim 1: $\measuredangle AOD= \measuredangle DOS$. Proof: It suffices to show $AOD\cong SOD$. Note that $\measuredangle ADO=\measuredangle ESO=\measuredangle SEO=\measuredangle SDO$ and $\measuredangle DAO=\measuredangle OED=\measuredangle OSD$. This implies $AOD\cong SOD$ claim 2: $SF\parallel BC$ proof: by claim 1, $\measuredangle AQD=\measuredangle AOD= \measuredangle DOS=\measuredangle DES=\measuredangle AFS$. to finish, consider the homothety mapping $PQ$ to $SF$, This maps $(APQ)$ to $(ABC)$, which implies that they are tangent at $A$
06.10.2024 20:35
Let $F=OD \cap BS$. Firstly, since $OA=OS$, $\measuredangle OAD = \measuredangle AEO = \measuredangle DSO$, and $\measuredangle ADO = \measuredangle EDO = \measuredangle ESO = \measuredangle OES = \measuredangle ODS$, we have $\Delta ADO \stackrel{-}{\cong} \Delta SDO$. It follows that $\measuredangle AOD = \measuredangle DOS$, so $\measuredangle AOF = \measuredangle DOS = \measuredangle AES = \measuredangle ABF$, so we have that $A,B,O,F$ are concyclic. Next, note that $\measuredangle QAB+ \measuredangle BAO = \measuredangle QAO = \measuredangle BDO = \measuredangle BFD + \measuredangle DBF = \measuredangle BAO + \measuredangle CAP$, so $AB,AC$ are isogonal in $\angle PAQ$. Extend $AB,AC$ to meet $(APQ)$ at $B',C'$, then it follows that $BC//B'C'$. Then we may take a homothety centered at $A$ taking $(ABC)$ to $(AB'C')$, so the two circles are tangent.