圆的切线是圆这部分内容中比较重要的内容,为此,本文介绍两种判定切线的常用方法,供同学们学习时参考.一、当直线(待定切线)与圆的公共点已明确时,则连结公共点与圆心得过公共点的半径,再证直线(待定切线)与此半径垂直. The tangent of the circle is the more important content of this part of the circle. For this reason, this paper introduces two common ways of determining the tangent, which are used for reference when the students learn. First, when the common point of the straight line (the tangent to be determined) and the circle is clear , then link the common point with the radius of the center point of the common point, and then verify that the straight line (to be tangent) is perpendicular to this radius.