Sn=n²-3n+1①s1=1-3+1=-1=a1当n≥2时,有S(n-1)=(n-1)²-3(n-1)+1=n²-5n+5②①-②得Sn-S(n-1)=n²-3n+1-(n²-5n+5)=2n-4=anan=2n-4n=1,a1=-1n≥2,an=2n-4