在立体几何中,当立体图形中量与量的关系不好理解时,常常通过它的平面展开图来理解;同样当平面图形能否围成立体图形不好确定时,也常常通过立体图形来判断.这种通过立体图形与它平面展开图来理解图形中的相关关系的方法是立体几何中常用的方法之一. In the three-dimensional geometry, when the relationship between quantity and quantity in a three-dimensional figure is not well understood, it is often understood through its flat surface development diagram; also when a flat figure cannot be established, it is often determined by a three-dimensional figure. Judgment. This method of understanding the correlation in a graph by using a three-dimensional graph and its plane expansion diagram is one of the commonly used methods in stereo geometry.