an=1/(3n-1)*(3n+2)=1/3*(1/(3n-1)-1/(3n+2))把1/3提出来sn=1/3(1/2-1/5+1/5-1/8+1/8-1/11+.......1/(3n-1)-1/(3n+2))=1/3(1/2-1/(3n+2))=n/2(3n+2）
