设等差数列{an}的前n项和为Sn,若S5=10,S10=30,则S15=_.

问题描述:

设等差数列{an}的前n项和为Sn,若S5=10,S10=30,则S15=______.

∵等差数列{an}的前n项和为Sn,若S5=10,S10=30,

5a1+
5×4
2
d=10
10a1+
10×9
2
d=30
,解得a1
6
5
,d=
2
5

S15=15a1+
15×14
2
d
=60.
故答案为:60.