（几何证明选讲选做题）已知PA是圆O的切线，切点为A，PA=2. AC是圆O

（几何证明选讲选做题）已知PA是圆O的切线，切点为A，PA=2.

AC是圆O的直径，PC与圆O交于点B，PB=1，则圆O的半径R= 。

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（几何证明选讲选做题）已知PA是圆O的切线，切点为A，PA=2. AC是圆O

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