直线与圆相切是《圆》一章中最重要的内容,也是学生在几何学习中感觉最困难的内容之一,它除了切线的判定外,主要由有关圆的切线的四个定理组成:切线的性质定理、切线长定理、弦切角定理、切割线定理。已知直线与圆相切的问题,若由四个定理人手解题,一般都方便有效。例1 如图(1),AT切⊙O于T,OA交⊙O于C,TB⊥OA于B,求证:TC平分∠ATB The tangency of the straight line and the circle is the most important part of the chapter “Circle”, and it is also one of the most difficult things for students to learn in geometry. In addition to the determination of the tangent line, it consists of four theorems about the tangent of the circle: Tangential property theorem, tangent length theorem, chord angle theorem, cutting line theorem. It is known that the problem of tangency between a straight line and a circle, if solved by four theorem hands, is generally convenient and effective. Example 1 As shown in figure (1), AT cuts O to T, OA crosses O to C, and TB to OA to B. Proof: TC split ATB