1
Sn=2n^2-n
S(n-1)=2(n-1)^2-(n-1)
an=Sn-S(n-1)
=2n^2-n-[2(n-1)^2-(n-1)]
=4n-3
an-a(n-1)=(4n-3)-[4(n-1)-3]
=4
是等差an=4n-3;
2.
Sn=2n^2 -n+1
a1=S1=2 -1+1=2
a2=s2-a1=7-2=5
S(n-1)=2(n-1)^2 -(n-1) +1
an=Sn-S(n-1)
=2n^2-n+1-[2(n-1)^2-(n-1)+1]
=4n-3
所以
a1=2
an=4n-3,(n≥2时)
即从第2项起成等差.