圆的切线的判定方法有三种：（1）和圆有唯一公共点的直线是圆的切线；（2）到圆心的距离等于半径的直线是圆的切线；（3）经过半径的外端并且垂直于这条半径的直线是圆的切线．方法（l）是从“定性”的角度去描述相切的特点，从而说明什么叫直线与圆相切；而具体问题一般只有 There are three methods for determining the tangent of a circle: (1) a straight line with a unique common point in a circle is a tangent of a circle; (2) a straight line with a distance equal to the center of the circle is a tangent to a circle; (3) passes through the outer end of the radius and The straight line perpendicular to this radius is the tangent of the circle. Method (l) is to describe the tangent characteristics from the “qualitative” point of view, so as to explain what is called the tangency of a straight line with a circle; and the specific problem is generally only