设数列首项a1，末项an，公差d，项数为，前n项和为Sn,Sn=na1+n(n-1)d/2知道了Sn、n和d，很容易求出a1=(Sn-n(n-1)d/2)/n=Sn/n-(n-1)d/2又有：an=a1+(n-1)d=Sn/n-(n-1)d/2+(n-1)d=Sn/n+(n-1)d/2
首项为a1,公差为d第n项为an=a1+（n-1)d
