D is the midpoint of the arc AB, so angle DOB = 1/2 angle AOB = angle ACB. Hence DO is parallel to AC. But OJ is parallel to AC, so AJOD is a parallelogram. Hence AJ = OD. Since EF is the perpendicular bisector of AO, we have AE = AF = OE, so A is the center of the circle through E, F and J. Hence angle EFJ = 1/2 angle EAJ. But angle EAJ = angle EAC (same angle) = angle EFC. Hence J lies on the bisector of angle EFC.
Since EF is perpendicular to AO, A is the midpoint of the arc EF. Hence angle ACE = angle ACF, so J lies on the bisector of angle ECF. Hence J is the incenter.
IMO 사이트에서 해답을 배겨왔습니다..-_-;;
그럼...
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해답을 찾으려고 해도 해답이 없어서 답답하네요