(1)sn=1/3(an-1)n=1 s1=a1=1/3(a1-1) a1=-1/2n=2 s2=a1+a2=1/3(a2-1) a2=-1/4(2)sn=1/3(an-1)s(n-1)=1/3(a(n-1)-1)sn-s(n-1)=an=1/3(an-a(n-1))an/a(n-1)=1/2所以数列{an}是等比数列 所以an=（-1/2）^(n-1)*(1/2)=-(...