一、理解圆的切线的判定定理经过半径的外端并且垂直于这条半径的直线是圆的切线.理解这个定理,必须抓住两点:(1)直线经过半径的外端点;(2)直线垂直于这条半径.这两个条件缺一不可.二、清楚切线的特征切线具有下列特征:(1)切线与圆只有一个公共点. First, understanding the decision of a round tangent theorem Theorem through the radius of the outer end and a straight line perpendicular to this radius is a tangent to understand this theorem, we must seize the two points: (1) straight line through the outer end of the radius; (2) The straight line is perpendicular to this radius. Both of these conditions are indispensable. Second, it is clear that the tangent feature tangent has the following characteristics: (1) The tangent line and the circle have only one common point.