Sn＝na1+n(n-1)d/2=－n^2
a1=-n-(n-1)d/2=(-1-d/2)n+d/2
-1-d/2=0(等差数列,a1定值,n不定,系数为0),d=-2
a1=d/2=-1
通项公式an=-1-2(n-1)=1-2n