1, fixed point: the common explanation is to choose or specify a place or a person who specializes in a certain job, or to specify a time;

2. The fixed point in mathematics refers to the point determined in the coordinate system;

3. In the unified definition of conic, the locus of a point whose distance ratio from a point to a fixed point and a fixed line is constant is called conic;

4, and this straight line is called the directrix;

5. In the general theory of spatial surfaces, surfaces can be regarded as trajectories formed by a family of curves moving along their directrix. For a surface generated by a curve family, the directrix is a spatial curve that intersects every curve in the curve family.