一个多面体被一个平面所截 ,在多面体的表面得到的截痕形成的平面封闭图形 ,称为这个多面体的一个截面 .判断截面有三项指标 :一是这个图形是否是平面图形 ;二是这个图形是否封闭 ;三是这个封闭图形的各条边是否在多面体的表面 .例如 ,图 1中的三角形就是正方体的一个截面 .在 A polyhedron is truncated by a plane, and the plane-closed pattern formed by the cut-off marks obtained on the surface of the polyhedron is called a section of this polyhedron. There are three indicators for judging the cross-section: whether the figure is a plane figure; and whether the figure is a figure Closed; third is whether the sides of the closed shape are on the surface of the polyhedron. For example, the triangle in Figure 1 is a section of the cube.