a
\(\frac{1}{{{f_1}}} = \left( {{\mu _1} - 1} \right)\left( {\frac{1}{R} - \frac{1}{\infty }} \right)\) ; \(\frac{1}{{{f_2}}} = \left( {{\mu _2} - 1} \right)\left( {\frac{1}{\infty } - \frac{1}{R}} \right)\)

\(\frac{1}{{{f_1}}} = \frac{{(\mu  - 1)}}{R}\) ; \(\frac{1}{{{f_2}}} =  - \left( {\frac{{{\mu _2} - 1}}{R}} \right)\)

\(\mathrm{f}_{2}=2 \mathrm{f}_{1}\)  ;  \(\frac{1}{\mathrm{f}_{1}}=\frac{2}{\mathrm{f}_{2}}\)

\(\frac{\mu_{1}-1}{R}=\frac{-2\left(\mu_{2}-1\right)}{R}\)

\(\mu_{1}+2 \mu_{2}=3\)