Cho nửa đường tròn (O), đường kính AB. Qua điểm C thuộc nửa đường tròn, kẻ đường thẳng (d) tiếp xúc với đường tròn. Gọi E và F là chân các đường vuông góc hạ từ A, B đến (d). Hạ CH vuông góc với AB. Chứng minh rằng: a) CE = CF. b) AC là tia phân giác của góc BAE. c) $CH^{2}$ = AE.BF

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