F(T_n)=\frac{\frac{1}{n^4}+\frac{1}{n^2}z_0^2+\frac{1}{n^2}z_0^2+\frac{1}{n^4}}{\frac{1}{n^2}+\frac{1}{n^2}} = \frac{\frac{2}{n^4}+\frac{2}{n^2}z_0^2}{\frac{2}{n^2}}=\frac{\frac{2+2n^2z_0^2}{n^4}}{\frac{2n^2}{n^4}}=\frac{1+n^2z_0^2}{n^2} \rightarrow z_0^2