Cho hình thang ABCD có hai đáy là AB và CD. Gọi M là trung điểm của CD, E là giao điểm của MA và BD, F là giao điểm của MB và AC. a) Chứng minh EF // AB b) Đường thẳng EF cắt AD, BC lần lượt tại H và N. Chứng minh HE = EF = FN c) Biết AB = 7,5 cm, CD = 12 cm. Tính độ dài HN.

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