$$\begin{gathered} If u=\infty v=f \\ If u=R=2f \\ \frac{1}{v}=2\frac{1}{f} \\ \frac{1}{v^2}=\frac{1}{{\left(2f\right)}^2}=b\mathrm{.......}\left(2\right) \\ \frac{a}{b}=4 \end{gathered}$$