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1 Cho tam giác ABC vuông tại A. Gọi O, D lần lượt là trung điểm của BC và AC. Đường thẳng vuông góc với OA tại A cắt tia OD tại H. Cho E là điểm di động trên đoạn AC. Qua H vẽ đường thẳng vuông góc với đường thẳng OE tại F. Chứng minh 4.OE + OF > hoặc = 2.BC 2 cho tam giác ABC vuông tại A,đường cao AH,AE vuông góc với AB tại E,Giả sử tam giác ABC vuông văn tại A,M là trung điểm của AC đường thẳng đi qua M vuông góc với BM cắt BC tại F,chứng minh BF=2FC
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