a3+a8=20
a1+a10=20
S10=?
求求你們了~
等差級數解答 拜託
2012-02-12 8:28 am
回答 (2)
2012-02-12 9:15 am
✔ 最佳答案
Sn= n(a1 + an)/2
S10
= 10 x (a1 + a10) / 2
= 10 x 20 / 2
= 100
參考: micatkie
2012-02-13 11:28 pm
快速解題法:
20 x 5 = 100
※ 20 = (a1+a10) = (a2+a9) = (a3+a8) = (a4+a7) = (a5+a6)
20 x 5 = 100
※ 20 = (a1+a10) = (a2+a9) = (a3+a8) = (a4+a7) = (a5+a6)
收錄日期: 2021-04-13 18:32:26
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20120212000016KK00248