810x811x812.x2011除以810的n次方(结果为正整数)n的最大值

[2011/3] - [809/3] = 670 - 269 = 401[670/3] - [269/3] = 223 - 89 = 134[223/3] - [89/3] = 77 - 29 = 48[77/3] - [29/3] = 25 - 9 = 16[25/3] - [9/3] = 8 - 3 = 5[8/3] - [3/3] = 2 -1 =1401+134+48+16+5+1=605...为什么要除以3？除以5？而且结果还要取近似数？请解答下，谢谢除以3取整，是求这些数中有多少3的倍数，就是计算结果可以分解出多少个3。