在立体几何中有这样的两类问题 ,一类是把平面图形按照一定要求进行折叠或旋转 ,得到空间形体 ,另一类是为解决某些问题 ,需要把空间图形展开为平面图形 .解决这两类问题的关键是要分清楚图形变化前后的位置关系和数量关系的变与不变 ,下面举例说明 .例 1　边长为 2ａ的正方形 There are two kinds of problems in the three-dimensional geometry, one is to fold or rotate the plane figure according to certain requirements to get the spatial shape, and the other is to solve some problems, it is necessary to expand the space graphic into a flat figure. The key to the two types of problems is to clearly distinguish between changes in the positional relationship and the quantitative relationship before and after the graphical change. Here is an example. Example 1 Squares with sides 2a