等差数列an中,若3n²+2n,则公差d=
问题描述：
等差数列an中,若3n²+2n,则公差d=
答
Sn=3n^2+2n
S(n-1)=3(n-1)^2+2(n-1)
=3n^2-6n+3+2n-2
=3n^2-4n+1
an=Sn-S(n-1)
=3n^2+3n-(3n^2-4n+1)
=3n^2+3n-3n^2+4n-1
=7n-1
d=7