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题目:已知如图,AB是⊙O的直径,BC是⊙O的切线,切点为B,OC平行于弦AD。求证:DC是⊙O的切线。 (人教版《几何》第三册P109)
Title: As known, AB is the diameter of ⊙O, BC is the tangent of ⊙O, the cut-off point is B, and OC is parallel to the chord AD. Proof: DC is the tangent of ⊙O. (People’s Education Edition “Geometry” Volume III P109)