Cho tứ giác lồi ABCD có 4 đỉnh thuộc đường tròn (O). Gọi Ha ,Hb ,Hc ,Hd lần lượt là trực tâm các tam giác BCD, CDA, DAB, ABC. Chứng minh rằng AHa ,BHb ,CHc ,DHd đồng quy.

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