sn=-n^2
s(n+1)=-(n+1)^2
a(n+1)=-(n+1)^2+n^2=(n-n-1)(n+n+1)=-2n-1=-2(n+1)+1
所以an=-2n+1
d=a(n+1)-an=-2n-1-(-2n+1)=-2