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圆与椭圆有着密切的关系:如果用一个平面截一个圆锥面,截得的图形可以是一个圆、椭圆、双曲线、抛物线或者两条相交直线,其中只有圆和椭圆是封闭曲线;再如在研究圆的投影时,当圆平面倾斜于投影面时,它在该投影面上的投影为一椭圆;最后“回到定义”看,到定点的距离等于定长的点的轨迹为椭圆,到两个定点的距离之和等于定长(大于定点的距离)的点的轨迹叫椭圆。
There is a close relationship between a circle and an ellipse: if a plane is used to cut a conical surface, the image can be a circle, an ellipse, a hyperbola, a parabola, or two intersecting lines, of which only the circle and the ellipse are closed curves; When studying a round projection, the projection of the circle plane on the projection plane is an ellipse when the circle plane is inclined to the projection plane. Finally, “Back to Definition”, the trajectory at a fixed point distance equal to the fixed length is an ellipse , The distance to the two fixed point is equal to the fixed length (greater than the distance from the fixed point) point trajectory called ellipse.