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Cho tam giác ABC cân tại A. Kẻ đường trung tuyến AN, G là trọng tâm a) Chứng minh: AN vuông góc với BC b) biết AC = 15cm, BC = 18cm. tính độ dài đoạn tahngr AN, AG c) Vẽ phân giác góc B cắt AN tại O. Từ O kẻ OH vuông góc AB, H thuộc AB. Chứng minh HN vuông góc OB. d) E là điểm trên tia phân giác ngoài góc C. chứng minh EA+EB>CA+CB
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