在圆锥曲线中 ,求弦长为定值的动弦中点的轨迹方程是解析几何中比较棘手的问题 ,解题的方法较多 ,但运算过程繁琐复杂 ,学生往往难以入手 .本文归纳一种解题方法———角参变量法 ,可以大大地减少计算量 ,简缩推理过程 .下面简述其解题的基本思想及解题规律 .设圆锥曲线Ｃ :Ｆ( In the conic section, finding the trajectory equation of the midpoint of the chord with a fixed chord length is a difficult problem in analytical geometry. There are many ways to solve the problem, but the calculation process is cumbersome and complex, and students often find it difficult to get started. The problem-solving method --- angle parameter method, can greatly reduce the amount of calculation, simplify the inference process. The following briefly describes the basic ideas and problem-solving law of its solution. Set the conic curve C: F (