an+Sn=n,a(n-1)+S(n-1)=n-1,前式减后式得：an-a(n-1)+an=1,2an-a(n-1)=1；2(an-1)=a(n-1)-1,(an-1)/[a(n-1)-1]=1/2,则数列{an-1}为等比数列公比q=1/2,因为a1+a1=1,首项为a1-1=-1/2,通项：an-1=-1/2*1/2^(n-1)=-1/2^n...