文[1]中给出了弦中点定理和逆定理,从而得到了求二次曲线弦族中点轨迹的简便方法.本文将利用此方法系统地讨论二次曲线的放射弦族中点轨迹.这一问题不仅本身饶有趣味,而且为我们用初等的方法研究二次曲线的切线奠定了基础. 过平面上一定点P的直线族被某二次曲线所截得的弦族,称为该二次曲线的、过点P的放 The string mid-point theorem and the inverse theorem are given in [1] to obtain a simple method to find the midpoint trajectory of the second-order curve family. In this paper, we will use this method to systematically discuss the midpoint trajectory of the family of quadratic curves. This problem is not only interesting in itself, but also lays the foundation for us to use the elementary method to study the tangent of the quadratic curve. The line family where a straight line of a certain point on the plane P is intercepted by a quadratic curve is called The quadratic curve, the point P put