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## Homework Statement

A biconvex lens, [itex]L_1[/itex], with focal length of magnitude [itex] \left|

f_1

\right|

= 15[/itex] cm is used with a second biconvex lens, [itex]L_2[/itex], with focal length of magnitude [itex] \left|

f_2

\right|

= 25[/itex] 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

[itex]\frac{1}{f_c} = \frac{1}{f_1} + \frac{1}{f_2}[/itex]

## 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

[itex]

f_1 > 0[/itex]

and

[itex]

f_2 < 0[/itex]?

Solution given is [itex] +9.375 [/itex] 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 [itex] +37.5 [/itex] cm