✔ 最佳答案
001: 請注意問題
2012-01-09 22:51:21 補充:
7^1 = 7
7^2 = 49
7^3 = 343
7^4 = 2401
7^5 = 16807
...
The cycle repeats for every 4 terms, i.e. 7,9,3,1,7,9,3,1,...
Sum of unit digit of every 4 terms = 7 + 9 + 3 + 1 = 20
Unit digit of 7^1 + 7^2 + 7^3 + ... + 7^1999 + 7^2000 + 7^2001 + 7^2002
= (0) + (0) + ... + (0) + Unit digit of 7^2001 + Unit digit of 7^2002
= Unit digit of (7 + 9)
= 6
參考: Knowledge is power.