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初三《几何》第三册课本中有个题:MN是⊙O的切线,CD是⊙O的直径,求证:点C、D与MN的距离和等于⊙O的直径.
There was a question in the third volume textbook of “Geometry” in the third grade: MN is the tangent line of ⊙O, CD is the diameter of ⊙O. Proof: The distance between point C, D and MN is equal to the diameter of ⊙O.