首先介绍有关正方体和正四面体的内切、外接球的几个结论:结论1设正方体的棱长为a,外接球半径为R1=3~(1/2)/2a,内切球半径为r_1=a/2.结论2利用“补形”方法把正四面体补成正方体(如图1),正四面体的外接球与补形后的正方体的外接球完全重合,设正四面体的棱长为b,外接 First of all, we introduce some conclusions about the inner and outer circumscribed sphere of cube and tetrahedron. Conclusions: 1 Suppose the cube of the cube is a, the radius of the external sphere is R1 = 3 ~ (1/2) / 2a, the radius of the inscribed sphere is r_1 = a / 2. Conclusions 2 The regular tetrahedron is filled into a cuboid by the method of “fill-in” (as shown in Fig. 1). The external quadrilateral of the regular tetrahedron coincides with the external quadrilateral The edge length of b, external