# Equivalent focal length of two lenses

## Homework Statement

Two thin lenses have focal lengths of -5cm and +20cm. Determine their equivalent focal lengths when (a) cemented together and (b) separated by 10cm.

## Homework Equations

##\frac{1}{f_{eq}}=\sum_{n=1}^k \frac{1}{f_n}##

## The Attempt at a Solution

So the first part was easy since its just with ##f_1=-5cm## and ##f_2=20cm##, ##f_{eq}=(\frac{1}{f_1}+\frac{1}{f_2})^{-1}=-6.667cm## but for the second part I'm not sure how to find it since there is separation in-between the two lenses. I thought it was given by the equation ##\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1f_2}## where ##d## is the distance between the lenses but this doesn't give the correct answer.

Edit: Solved it

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