a(n+2)-a(n+1)=(1/3)[a(n+1)-a(n)],{a(n+1)-a(n)}是首项为a(2)-a(1)=7/9 - 1/3 = 4/9,公比为(1/3)的等比数列.a(n+1)-a(n) = (4/9)(1/3)^(n-1) = 4/3^(n+1),a(n+1)3^(n+1) = 3a(n)3^(n) + 4,2+a(n+1)3^(n+1) = 3[2 + ...