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=______.
答
设等差数列的首项为a1,公差为d,则
=
S7−S3
S10
=4a1+18d 10a1+45d
=2 5
⇒S10=20;8 S10
同理
=
Sn−Sm
Sn+m
=(n−m)•(a1+
d)n+m−1 2 (n+m)a1+
d(n+m)(n+m−1) 2
=n−m n+m
⇒Sn+m=a Sn+m
•a.n+m n−m
故答案为:20,
•an+m n−m