某等差數列的首n項和為n^2,求該數列的第10項。
a. 19
b. 21
c. 28
d.31
e.100
等差數列及等比數列
2007-01-28 6:09 am
回答 (2)
2007-01-29 1:00 am
✔ 最佳答案
sum of first 10 terms = 10^2 = 100sum of first 9 terms = 9^2 = 81
so the 10th term = 10^2 - 9^2 = 19
So answer is A
No need to find first term and common difference!!!
2007-01-28 6:14 am
某等差數列的首n項和為n^2,求該數列的第10項。
設首項是a﹐公差是d
首n項和=n/2[2a+(n-1)d]=n^2
代n=1
a=1
代n=2
2a+d=4
d=2
第10項
=a+(10-1)d
=a+9d
=1+9*2
=19
答案是a
設首項是a﹐公差是d
首n項和=n/2[2a+(n-1)d]=n^2
代n=1
a=1
代n=2
2a+d=4
d=2
第10項
=a+(10-1)d
=a+9d
=1+9*2
=19
答案是a
收錄日期: 2021-04-22 00:30:34
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https://hk.answers.yahoo.com/question/index?qid=20070127000051KK04635