(1-1/3)(1+1/3)(1+1/9).（1+1/3^(2n))/(1-1/3)=[1-(1/3)^(4n)]/(1-1/3)=3/2*[1-(1/3)^(4n)]
先乘一个因式(1-1/3) 然后除以一个(1-1/3)