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cho ΔABC cân tại A, gọi M là trung điểm BC. từ M kẻ ME vuông góc với AB tại E, MF vuông góc với AC tại F. chứng minh a) ΔBEM = ΔCFM b) ΔAEM = ΔAFM c) ΔAEC = ΔAFC d)chứng minh AM ⊥EF.
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