1Sn=n²+nSn-1=(n-1)²+(n-1) (n≥2)Sn-Sn-1=n²+n-(n-1)²-(n-1)=2n当n=1时S1=a1=2满足an=2n 2a3a7=a5²a5=±8∵an>0∴a5=8
