Cho đường tròn (O;R) đường kính AB . Qua M thuộc đường tròn vẽ tiếp tuyến đường tròn cắt tiếp tuyến tại A và B của đường tròn lần lượt tại C và D a) Cm: AC + BD = CD và GÓC COD = 90° b) Gọi N là giao điểm BC và AD. Cm: MN vuông AB

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