an=4-3a(n+1)
an-1=-3(a(n+1)-1)
设bn=an-1
b1=8
bn=8/(-3)^(n-1)
Sbn=Sn-n
Sbn=6(1-(-1/3)^n)
Sbn-6=-6*(-1/3)^n
｜Sn-n-6|=6*(1/3)^n
|Sn-n-6|<1/125
6(1/3)^n<1/125
(1/3)^n<1/750
3^n>750
因:3^6=729,3^7=2187
故:n最小=7