1引言切线概念有着悠久的历史.公元前3世纪,古希腊数学家欧几里得在《几何原本》第3卷中将圆的切线定义为“与圆相遇,但延长后不与圆相交”的直线[1].同卷中有关切线的各命题表明,欧几里得是从以下几个角度来看圆的切线的:切线与圆的公共点个数为1;切线不能穿过圆或圆位于切线的同一侧;切线与圆心至切点的连线垂直.之后,阿波罗尼斯在《圆锥曲线》中将圆的切线定 1 Introduction The tangent concept has a long history. In the 3rd century BC, Euclid Euclid, an ancient Greek mathematician, defined the tangent of a circle in Volume 3 of the original “Geometry” as “meeting a circle but not extending after ”The same paper on the tangent of the various propositions show that Euclidean is a tangent of the circle from the following points:  tangent and the circle of the common point number is 1;  tangent can not Through the circle or circle is located on the same side of the tangent;  tangent and the center of the circle to the tangent point perpendicular to the line after Apollonius in the “conic” will be tangent of the circle set