以下三道关于抛物线弦中点的轨迹问题引起了我的思考 ,即 :例 1　直线ｌ过抛物线 ｙ2 =4ｘ的顶点 ,与抛物线相交所得的弦为ＰＱ ,求ＰＱ的中点Ｍ的轨迹方程 .例 2　直线ｌ过抛物线 ｙ2 =16ｘ的焦点 ,与抛物线相交所得的弦为ＰＱ ,求ＰＱ的中点Ｍ的轨迹方程 .例 3　直线ｌ过 (0 The following three trajectory questions about the midpoint of the parabola string caused my thinking, namely: Example 1 The line l passes through the vertex with y2 = 4x, and the line that intersects the parabola is PQ. Find the trajectory equation of the midpoint M of PQ. Example 2 The line l passes the focus of the parabola y2 =16x, and the parabola intersects the resulting string as PQ. Find the trajectory equation of the midpoint M of PQ. Example 3 Line l Over (0