Please give detailed solution!
更新1:
* us --> is
Geometric sequence
2011-05-05 5:12 am
Given a geometric sequence whose sum of the first 10 terms us 4 and whose sum from the 11th to the 30th is 48, find the sum from the 31st to the 60th term.
Please give detailed solution!
Please give detailed solution!
回答 (1)
2011-05-05 7:59 am
✔ 最佳答案
SUM OF GEOMETRIC SEQUENCE= a(r^n - 1)/(r - 1)
So,
a(r^10 - 1)/(r - 1) = 4...(1)
ar^10(r^20 - 1)/(r - 1) = 48...(2)
So, 4/(r^10 - 1) = 48/[r^10(r^20 - 1)]
r^10(r^10 + 1) = 12
r^10 = 3
sum from the 31st to the 60th
= ar^30(r^30 - 1)/(r - 1)
= 2*27*26
= 1404
=
收錄日期: 2021-04-26 14:07:07
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https://hk.answers.yahoo.com/question/index?qid=20110504000051KK01072