已知数列{an}的前n项和Sn=n^2-9n,则其通项an=多少;若它的第k项满足5<ak<8,则k=多少
人气:218 ℃ 时间:2020-01-28 07:35:53
解答
Sk=k^2-9k
S(k-1)=(k-1)^2-9(k-1)=k^2-11k+10
ak=Sk-S(k-1)=k^2-9k-(k^2-11k+10)
=2k-10
所以5
推荐
- 已知数列{an}的前n项和Sn=n^2-9n,第k项满足5<ak<8,则k等于
- 已知数列{an}的前n项和sn=n²-9n(1)求通项公式an(2)若5
- 数列【An】的前n项和Sn=n^2-9n,第K项满足Ak大于5小于8,则K=?
- 已知数列{An}的前n项和Sn等于n平方减9n,第k项满足5<Ak
- 已知数列{an}的前n项和Sn=n2-9n,第k项满足5<ak<8,则k等于( ) A.9 B.8 C.7 D.6
- To suggest that the student did not do the reading
- -lg5 -lg7 怎么化成1\5 1\7
- at ease,be likely to,in general,lose face,defend aganist,turn one's back to用五句话编一个故事
猜你喜欢