设首项为a,公差为d,则：
Sn=a+(a+d)+……+(a+(n-1)d)
=【a+(a+(n-1)d)】/2*n
=0.5*(n-1)*n*d+an
=(0.5d)*n^2+(a-0.5d)*n
当且仅当d=0,a=0时,前n项和Sn有极限.