2a & gt2c, an ellipse.
2a=2c, which is a line segment with two fixed points as endpoints.
2a & lt2c, no trace.
(2) The difference between the distances to the fixed points is 2a, and the distance between two fixed points is 2c.
2a & lt2c, a hyperbola.
2a=2c, which are two rays starting from two fixed points.
2a & gt2c, no trace.
(3) The distance from a fixed point is a, and the distance from a point to a straight line is d..
The fixed point is not a straight line, but a parabola.
A fixed point on a straight line is a straight line.