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KA1509
2 năm trước
cho đoạn thẳng BC cố định có độ dài bằng 2a (với a>0 ) và một điểm A di chuyển sao cho góc BAC =90 độ. Kẻ AH vuông góc với BC tại H. Gọi HE và HF lần lượt là đường cao của tam giác ABH và tam giác ACH a, Chứng minh BC^2= 3.AH^2 +BE^2 +CF^2 b, tìm điều kiện của tam giác ABC để tổng BE^2 + CF^2 tạo giác trị nhỏ nhất
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