Question

等差数列{An}的前n项和为40,前2n项和为120,则前3n项和为_____

Answer 1

Sn,S2n-Sn,S3n-S2n是等差数列
即40,120-40,S3n-120是等差数列
2×80=40+S3n-120
S3n=240

Answer 2

S(k)，S(2k)-S(k)，S(3k)-S(2k)为等差数列则
S(3n)-S(2n)=120
则S(3n）=240