Cho đường tròn (O), dây AB cố định không đi qua tâm O. Lấy điểm M thuộc tia đối của tia BA. Kẻ tiếp tuyến ME,MF với E, F thuộc đường tròn(O), gọi H là trung điểm của AB. 1, Chứng minh 5 điểm O,H,E,M,F cùng thuộc một đường tròn. 2, Gọi I, K lần lượt là giao điểm của EF với OH, OM. Chứng minh OH×OI=OK×ON. 3, Chứng minh IA là tiếp tuyến của đường tròn.

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