Cho tam giác ABC cân tại A. Trên tia đối tia BC lấy điểm D, trên tia đối của tia CB lấy điểm E sao cho BD=CE. Kẻ BH vuông góc với AD (H thuộc AD), kẻ CK vuông góc với AE (K thuộc AE). Chứng minh: a, Tam giác ABD= tam giác ACE b, BC//HK c, Gọi M, N, P, Q thứ tự là trung tâm của AD, AB, AC, AE. Chứng minh bốn đường thẳng CN, BP, DQ, EM cùng đi qua một điểm

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