等比等差的問題 20點
回答 (2)
2008-09-21 10:03 pm
✔ 最佳答案
1. 7*14 = 987*15 = 105
...
7*142 = 994
7*143 = 1,001
所有可被 7 整的三位數之和
= 7*15 + 7*16 + ... + 7*142
= 7* (15+16+...+142)
= 7 * (15 + 142 ) * (142 - 15 +1)/2 (等差數列和為頭加尾乘項數除2)
= 70,336
2008-09-21 10:57 pm
thank you so much!
收錄日期: 2021-04-24 00:16:01
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20080921000051KK01202