Cho tứ giác MNPQ có NP=MQ và NPkhông song song với MQ. gọi A, B, C, D, E, F lần lượt là trung điểm của các đoạn thẳng MN, NP, PQ. QM, MP,NQ. a (1,25đ) Chúng minh tứ giác AFCE là hình thoi. b'(1,25đ)Chúng minh các đoạn thẳng AC, BD, EF cùng cắt nhau tại một điểm. d (0,5đ) Tim thêm điều kiện của tứ giác MNPQ để B,EF,D thẳng hàng.

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