## Fundamentals of Physics Extended (10th Edition)

$f=+8.0cm$
We solve the lens equation $$\frac{1}{f}=\frac{1}{i}+\frac{1}{p}$$ to solve for focal length $f$: $$\frac{1}{f}=\frac{p}{ip}+\frac{i}{ip}$$ $$\frac{1}{f}=\frac{i+p}{ip}$$ $$f=\frac{ip}{i+p}$$ Substituting known values of $i=+12cm$ (see answer 18e for explanation) and $p=+24cm$ yields a focal length of $$f=\frac{(+12cm)(+24cm)}{12cm+24cm}=+8.0cm$$