等差数列an的前几项和为Sn,已知S3=6,a1=11,则公差d等于
问题描述:
等差数列an的前几项和为Sn,已知S3=6,a1=11,则公差d等于
a1=4
答
设公差为d,则S3=3a1+3d=6,a3=a1+2d=8,解之可得:a1=-4,d=6 S12-S9=a12+a11+a10=3a1+30d=168 a3=a1+2d=8,s3=3a1+3d,