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cho tam giac abc ,M la trung diem cua BC,duong phan giac ngoai cua goc A cat duong thang BC tai D.Duong tron ngoai tiep tam gia ADM cat tia Ab tai E, cat tia doi cua tia AC tai F. Goi N la trung diem cua EF. chung minh MN song song voi AD