Cho ba điểm A, B, C thẳng hàng (B nằm giữa A và C) vẽ đường tròn tâm O đường kính BC; AM là tiếp tuyến vẽ từ A (M là tiếp điểm). Từ M vẽ dây MN vuông góc với BC tại H. a) Chứng minh MB là tia phân giác của góc AMN; b) Từ B vẽ đường thẳng song song với MC, đường thẳng này cắt MN, MA lần lượt tại D và E. Chứng minh: AB.HC = AC. HB
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