Question

等差数列的前n项和为Sn，若a6=S3=12，求公差d和通项公式an

Answer 1

a1^2+2a1d+d^2=2a1^2+2a1d+2a1 a1^2+2a1-d^2=0 s3=3a1+3*an=a1+(n-1)*d =1.6+2.4n-2.4 =2.4n-0.8

追问

看不懂啊

Answer 2

s3=a1+a2+a3=3a1+3d=12
a1+d=4
a6=a1+5d=12
4d=8
d=2
an=a1+2(n-1)

追问

d=3 吧？？？

追答

d=2