Cho hình vuông ABCD, AC và BD cắt nhau tại O. Gọi M là trung điểm của cạnh BC, N là trung điểm của cạnh CD. AN cắt DM tại I. Chứng minh rằng: a) AN vuông góc DM và DO^2 = DI.DM b) BI = BA và góc IBC = 2 góc ICD c) IM là phân giác của góc OIC. MN ƠI GIÚP EM cm DO^2=DI.DM và câu b vs mn

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