f(1)=1
n>=2时,1/ f(n) = 1/3* [1/(n-1) - 1/n] }
1/（f1）+1/（f2）+1/（f3）+……+1/（fn）< 1+1/3*(1-1/n) < 4/3 = 4/3 *1 = 4/3f(1)