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Cho Δ ABC cân tại A, có BC = 2a. M là trung điểm của BC. Lấy D,E theo thứ tự thuộc các cạnh AB,AC sao cho góc DME = góc B a) Chứng minh tích BD.CE không đổi. b) Chứng minh DM là phân giác góc BDE. c) Tính chu vi Δ AED nếu Δ ACB đều. Mọi người giúp em với ạ T^T
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