多球相切问题在各类竞赛中经常出现 ,但由于作图复杂 ,给分析解决问题带来困难 .如果能透过现象 ,抓住问题的本质 ,将其转化为多面体问题 ,常能顺利解决 ,请看以下几例 .例 1　 (2 0 0 2年“希望杯”试题 )将 3个半径为 1的球和一个半径为 2 -1的球叠为两层放在桌面上 ,上层只? Multi-sphere tangential problems often appear in various competitions, but due to the complexity of drawing, it is difficult to analyze and solve problems. If we can grasp the nature of problems through phenomena, we can turn them into polyhedral problems, and we can often solve them successfully. , Please see the following examples. Example 1 (2002 “Hope Cup” questions) Put three balls with a radius of 1 and a ball with a radius of 2-1 on the desktop. The upper layer only?