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Tungtom
1 năm trước
cho tam giác ABC có ba góc nhọn nội tiếp đường tròn tâm O. Các đường cao AD, BK của tam giác gặp nhau tại H. Gọi E, F theo thứ tự là giao điểm thức hai của BO và BK kéo dài với đường tròn (O). a.Chứng minh EF song song AC b. Gọi I là trung điểm của AC. Chứng minh 3 điểm H,I,E thẳng hàng và OI=1/2BH
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