a(n+1)-2/3=0.5an+1/3-2/3
a(n+1)-2/3=0.5an-1/3
a(n+1)-2/3=0.5(an-2/3)
(a(n+1)-2/3)/(an-2/3)=1/2
因此：{an-2/3}是等比数列
a1-2/3=5/12
an-2/3=(1/2)^(n-1)*5/12
所以 an=(1/2)^(n-1)*5/12+2/3