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maithuythuong
2 năm trước
Cho (O;R) và điểm A cố định sao cho OA = 2R. Từ A kẻ 2 tiếp tuyến AB, AC và cát tuyến AMN với đường tròn (B ,C, M,N thuộc đường tròn và AM < AN). Gọi D là trung điểm của MN ; CD kéo dài cắt (O) tại E. a) 5 điểm A, B, O ,D ,C cùng nằm trên 1 đường tròn b) OA vuông góc BC tại H và tính S tam giác OBC c) BE // MN Giúp mình với
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