# Homework Help: Lens equation: pretty simple probably

1. Jan 8, 2010

### johnson.3131

1. The problem statement, all variables and given/known data

A converging lens ( f = 10.4 cm) is held 8.80 cm in front of a newspaper, the print size of which has a height of 2.04 mm. Find the image distance (in cm) and the height (in mm) of the magnified print.

D= _____cm
H= _____cm

2. Relevant equations

1/f=1/do+1/di

3. The attempt at a solution

1/10.4=1/8.80+1/di

I know this is the right setup but i feel like an idiot because I forget basic math...I don't remember how to solve this equation so if something can walk me through it step by step that would be great. I only have a simple scientific calculator that I keep using and get a decimal that isn't that right answer so i just don't know

2. Jan 8, 2010

### tiny-tim

Hi johnson.3131!
Well, there's only two steps â€¦subtract 1/8.80 from 1/10.4, and then 1/x it.

If you're getting it wrong, perhaps you'd better show us what you've done.

(you did remember the height is in mm, not cm?)

3. Jan 8, 2010

### Stonebridge

this is correct
1/10.4=1/8.80+1/di
so
1/10.4 - 1/8.8 = 1/di

Your calculator should have a 1/x function to calculate the two fractions on the left and subtract the answers. (The answer will be negative).
Then take that answer and use 1/x on it to get the value of di. (also negative)