要证明一条直线是圆的切线,我们可以应用切线的判定定理:“经过半径外端并且垂直于这条半径的直线是圆的切线。”这个定理的题设有两个部分:①此直线必须经过半径外端点;②此直线必须与这条半径垂直。证明一条直线是圆的切线时,一定要注意考虑这两个方面,看下面例子: To prove that a straight line is a tangent of a circle, we can apply the tangent decision theorem: “A line passing through the outer end of the radius and perpendicular to this radius is a tangent to the circle.” The theorem has two parts: 1 This line must be After the endpoint of the radius; 2 This line must be perpendicular to this radius. When you prove that a straight line is a tangent of a circle, you must pay attention to these two aspects. See the following example: