切线是初中几何教材中比较重要的内容，中招考试中也占有相当的比重，对学生学习来说也是一个难点．当直线和圆有惟一公共点时，直线和圆相切．这是直线和圆相切的定义，也是判断直线和圆相切的重要方法．本文再介绍两种证明切线问题的常用方法，以供参考．一、圆心到直线的距 Tangent is a more important content in junior middle school geometry textbooks. It also occupies a considerable proportion in the middle of the exam. It is also a difficult point for students to learn. When a straight line and a circle have a unique common point, the straight line and the circle are tangent. This is a definition of the tangency of a straight line and a circle. It is also an important method for determining the tangency of a straight line and a circle. This article describes two common methods for proving tangent problems for reference. First, the distance from the center to the straight line