平面上 為銳角三角形,其外心為 ,外接圓為 。分別在線段 上各取一點 ,並作過 與 垂直的直線 。設 分別與三角形 的外接圓及 再交於點 。令直線 與 交於點 ,直線 與 交於點 ,且點 為三角形 的垂心。
試證: 四點共圓。
, 2022
Solution
Let be points on lines , respectively, such that is parallel to , and passes through . From
we see that are concyclic. Since
are concyclic. Therefore, .
Let be the intersections of and , , respectively. Then
gives the concyclicity of . From the similarity , we see that is parallel to . According to Reim's theorem, are concyclic. Thus,
as desired.
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