Mathematical definition of orthographic projection: in the parallel projection of an object, the projection line is perpendicular to the projection plane, then this parallel projection is called orthographic projection. ? Its characteristics: 1. Is the orthographic projection of a line or segment perpendicular to the projection plane a point? 2. The orthogonal projection of a plane figure perpendicular to the projection plane is a straight line or a part of a straight line.

In this problem, A and AB are greater than or equal to CD.

The process of solving the problem is shown in the figure. It can be seen from the figure that when AB is perpendicular to the projection plane, CD is a point with a length of 0; When AB is parallel to the projection plane, CD=AB. So AB≥CD.