等差数列的前n项和为Sn,若S7-S3=8,则S10=_;一般地,若Sn-Sm=a(n>m),则Sn+m=_.

问题描述:

等差数列的前n项和为Sn,若S7-S3=8,则S10=______;一般地,若Sn-Sm=a(n>m),则Sn+m=______.

设等差数列的首项为a1,公差为d,则

S7S3
S10
4a1+18d
10a1+45d
2
5
8
S10
S10=20;
同理
SnSm
Sn+m
(n−m)•(a1+
n+m−1
2
d)
(n+m)a1+
(n+m)(n+m−1)
2
d
=
n−m
n+m
a
Sn+m
Sn+m
n+m
n−m
•a

故答案为:20,
n+m
n−m
•a