a(n+1)=(1/3)an+(1/2)^(n+1)
a(n+1)-3(1/2)^(n+1) = (1/3)[an - 3(1/2)^n ]
{an - 3(1/2)^n} 是等比数列,q= 1/3
an - 3(1/2)^n = (1/3)^(n-1) .{a1 - 3(1/2)^1}
= -2(1/3)^n
an =3(1/2)^n - 2(1/3)^n