a
\(\frac{f_{1}}{f_{2}}=-\frac{\omega_{1}}{\omega_{2}}=-\frac{1}{2} \quad \therefore f_{2}=-2 f_{1}\)

As \(\frac{1}{\mathrm{F}}=\frac{1}{\mathrm{f}_{1}}+\frac{1}{\mathrm{f}_{2}}\)

\(\therefore \frac{1}{20}=\frac{1}{\mathrm{f}_{1}}-\frac{1}{2 \mathrm{f}_{1}}=\frac{1}{2 \mathrm{f}_{1}} \quad \therefore \mathrm{f}_{1}=10 \mathrm{\,cm}\)

\(f_{2}=-20 \mathrm{\,cm}\)