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cho tam giác ABC . Trên tia đối của các tia AB , AC lần lượt lấy các điểm D , E sao cho AD = AB và AE = AC a) chứng minh DE // BC b) gọi M , N lần lượt là trung điểm của BC và DE . Chứng minh A là trung điểm của MN
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