复杂的排列组合问题常常通过实验、画简图、小数学简化手段使问题直观化,从而寻求解题途径,由于结果的正确性难以直接验证,因而常用不同的方法来获得检验,有时选准思考角度解组合问题,会感到很轻松.　　一、插空法例1 　马路上有编号为 1,2,3,…,9 的 9只路灯,为节约用电, The complex permutation and combination problems often make the problems visible through experiments, drawing diagrams, and small mathematical simplifications, so as to seek solutions to the problem. Because the correctness of the results is difficult to verify directly, different methods are commonly used to obtain inspections. It is easy to solve the problem of combination and perspective. First, insert the legislation 1 There are 9 street lamps numbered 1,2,3,...,9 on the street to save electricity.