Sn,S2n,S3n,成等差数列，2*S2n=Sn+S3nS3n=72补充，像S1,Sk,S2k,S3k,下标成等差数列，则这三个数成等差数列
S3n+Sn=2S2n S3n=2S2n-Sn=2*60-48=72
