>
数学
>
设S
n
是等差数列{a
n
}的前n项和,a
12
=-8,S
9
=-9,则S
16
=______.
人气:435 ℃ 时间:2020-06-17 11:40:50
解答
S
9
=
1
2
(a
1
+a
9
)×9=-9,又有a
1
+a
9
=2a
5
,
可得,a
5
=-1,
由等差数列的性质可得,a
1
+a
16
=a
5
+a
12
,
则S
16
=
1
2
(a
1
+a
16
)×16=
1
2
(a
5
+a
12
)×16=-72.
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