已知等差数列an前n项和为sn，s3=60,sn=300,sn-3=180,则n=?

由S3=60得：a1+a2+a3=60 (1)由Sn-Sn-3=300-180=120得：an+an-1+an-2=120 (2)(1)+(2)得：a1+a2+a3+an+an-1+an-2=180 (a1+an)+(a2+an-1)+(a3+an-2)=180 a1+an=a2+an-1=a3+an-2 即：3(a1+an)=180 a1+an=60sn=n(a1+an)/2=60n/2=300 n=10