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August 29, 2014

Homework Help: math

Posted by Reiny on Wednesday, May 9, 2007 at 5:09pm.

You did not watch your units, you are mixing inches and feet

Your equation should have been
1/2 = 1/di + 1/120 (I changed 10 feet to 120 in)

solve this for di and you get 2.03 inches.

Please help me I am so confused I think I am losing my mind.


The focal length f of a camera lens is 2 inches. The lens equation is 1/f=(1/di)+(l/do), where di is the distance between the lens and the film and do is the distance between the lens and the object.

The object to be photographed is 10 ft away. How far should the lens be from the film?

my work:
1 1 1
- = - + -
f di do

1 1 1
- = - + -
2 di 10

You subtract 1/10 and get 4/10=1/di. di=2.5

but supposedly the answer is 2.03. How?

1 1 1
- = - + -
f di do

if you can't tell what i mean, this is it: 1/f=(1/di)+(1/do)

Lol, trick question! Thank you!

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