a(n+1)=1/2(an-a(n+1))
2*a(n+1)=an-a(n+1)
3*a(n+1)=an
a(n+1)=(1/3)*an
故an为等比数列,q为1/3;
因a4=2/27,所以a1=2,an=(1/3)^(n-1)*2;
Sn=2*(1-(1/3)^n)/(1-1/3)=3-(1/3)^(n-1);
a(n+1)=1/2(an-a(n+1))
2*a(n+1)=an-a(n+1)
3*a(n+1)=an
a(n+1)=(1/3)*an
故an为等比数列,q为1/3;
因a4=2/27,所以a1=2,an=(1/3)^(n-1)*2;
Sn=2*(1-(1/3)^n)/(1-1/3)=3-(1/3)^(n-1);