# Determining the focal length of a compound lens

## Homework Statement

A biconvex lens, $L_1$, with focal length of magnitude $\left| f_1 \right| = 15$ cm is used with a second biconvex lens, $L_2$, with focal length of magnitude $\left| f_2 \right| = 25$ cm to form a compound lens system.

If the two lenses are placed in mutual contact what is the focal length of the compound lens?

## Homework Equations

$\frac{1}{f_c} = \frac{1}{f_1} + \frac{1}{f_2}$

## The Attempt at a Solution

It's just a matter of plugging into the above equation.. However do I use the the absolute values or the fact that for a biconvex lens

$f_1 > 0$

and

$f_2 < 0$?

Solution given is $+9.375$ cm, which suggests you use the absolute values, however I am not fully convinced as it was not specified in the course notes whether to use absolute values or not.

Not using absolute values, I find the focal length to be $+37.5$ cm