I have the equations in front of me...

1)   f=c/2

2)   1/f=1/do+1/di  

3)   hi/ho=di/do

I know 1)...
but what are 2) and 3) used for?

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Help me get to college!!!

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i've never seen those shorthands used b4. Can u give some more info?

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f=c/2
F= Focal Length
C= Location of the object

1/f=1/Do+1/Di
f= Focal Length
Do= Distance of Object
Di= Distance of Image

Hi/Ho=Di/Do
Hi=Height of Image
Ho=Height of Object
Di= Distance of Object
Di= Distance of Image

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Help me get to college!!!

"Hi / Ho = Di / Do

We recognize this as the magnification equation stating the ratio of image size to object size."

and:

"1/f = 1/Do + 1/Di

This equation is exactly the same as the one for convex lenses. However, when applying it to concave lenses, we note that the focal length must always be considered negative, and the arithmetic will generally result in Di being negative also."

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