>
数学
>
设数列{a
n
}的前n项和为S
n
,S
n
=
a
1
(
3
n
−1)
2
(对于所有n≥1),且a
4
=54,则a
1
的数值是______.
人气:260 ℃ 时间:2020-04-14 09:43:31
解答
设数列{a
n
}的前n项和为S
n
,S
n
=
a
1
(
3
n
−1)
2
(对于所有n≥1),
则a
4
=S
4
-S
3
=
a
1
(81−1)
2
−
a
1
(27−1)
2
=27
a
1
,
且a
4
=54,则a
1
=2
故答案为2
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