切线的判定方法是近年来中考命题的热点 .根据切线的定义和判定定理 ,切线的判定方法有如下两种 .方法 1　已知直线和圆有一个公共点时 ,应作出过公共点的半径 ,然后证明直线垂直于过公共点的半径 .例 1　如图 1 ,ＡＢ是⊙Ｏ的直径 ,ＢＣ是⊙Ｏ的切线 ,Ｂ为切点 ,ＯＣ平行于弦 The method of judging the tangent is the hot spot of the proposition in recent years. According to the definition of the tangent and the determination theorem, there are two methods for determining the tangent. Method 1 When the straight line and the circle have a common point, the radius of the common point should be made. Then prove that the straight line is perpendicular to the radius of the common point. Example 1 As shown in Figure 1, AB is the diameter of ⊙O, BC is the tangent of ⊙O, B is the cut point, and OC is parallel to the string.