4-6bn+3b(n+1)=0
b(n+1)=2bn-4/3
b(n+1)-4/3=2(bn-4/3)
thus:bn-4/3公比为2的等比数列
bn-4/3=(2/3)*2^(n-1)
bn=(2/3)*2^(n-1)+4/3