Cho tâm giác nhọn ABC nội tiếp đường tròn tâm O, có góc BAC=60°. Hai đường cao BB' và CC' của tam giác ABC cắt nhau tại H. Chứng minh rằng: a) Bốn điểm B, H, O, C cùng thuộc một đường tròn. b) Hai góc BAC và góc HAO có chung đường phân giác. c) Hai đường thẳng OA và B'C' vuông góc.

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