学习了切线的判定定理后，善于总结的同学会发现判定切线的方法有以下三种：（l）和圆只有一个公共点的直线是圆的切线；（2）过半径外端且和半径垂直的直线是圆的切线；（3）与圆心的距离等于半径的直线是圆的切线．其中（l）是切线的定义，（2）和（3）的本质是相同的，只是 After learning the tangent decision theorem, there are three ways in which students who are good at summing up can find out the tangent: (l) The circle with only one common point is the tangent of the circle; (2) The outer end of the radius is perpendicular to the radius. The straight line is the tangent of the circle; (3) The straight line whose distance from the center is equal to the radius is the tangent of the circle. Where (l) is the definition of the tangent, and the essence of (2) and (3) is the same, only