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（几何证明选讲选做题）如图，圆 O 的割线 PBA 过圆心 O，弦 CD 交 PA 于点F，且△COF∽△PDF，PB = OA = 2，则PF = 。

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（几何证明选讲选做题）如图，圆 O 的割线 PBA 过 圆心 O，弦 CD

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（几何证明选讲选做题）如图，圆 O 的割线 PBA 过圆心 O，弦 CD