Cho tam giác ABC cân tại A, điểm D bất kì nằm giữa B và C. vẽ DE//AB, DF//AC(E thuộc AC, F thuộc AB) a) chứng minh tổng DE + DF có giá trị ko đổi khi D di động b) Gọi M, N lần lượt là trung điểm của AB, CD. Chứng minh MN đi qua trung điểm của EF Có vẽ hình nha, thanks nhìu

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