# Lenses Question

#### Grekory

When i'm given do, ho, and hi. How would i solve for di
Normally i would do -di/do = hi/ho and cross-multiply but i'm not sure how the negative comes into effect.

do = object distance
di = image distance
ho = object height
hi = image height

If...

ho = 3cm
hi = 4cm
do = 5cm

What would be di?

Thanks

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#### Doc Al

Mentor
Please provide the complete statement of the problem you are trying to solve. (I presume that a negative image height means that the image is inverted.)

#### Grekory

It's just made up values and i want to know how i would solve for di. The negative in the formula confuses me

#### Doc Al

Mentor
Start with:
-di/do = hi/ho

You can solve for di by cross-multiplying if you like, or just multiply both sides by -do. As I said before, a negative value for di just means that the image is inverted. (Real images are inverted.)

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