已知{an}是等差数列,若Sn=20,S2n=38,求S3n的值

问题描述:

已知{an}是等差数列,若Sn=20,S2n=38,求S3n的值

S2n=Sn+Sn+nd=38=20+20+nd
∴nd=-2
S3n=S2n+Sn+2nd=56