Solving Image Projection Problem | Lens Focal Length Calculation

[yeyspaz]

"A 35 mm slide (picture actually 24*36 mm) is to be projected on a screen 1.8*2.7 m placed 9 m from the projector. What focal length lens should be used if the imageis to cover the screen?"

I was trying to do this problem using the formula (1/do)+(1/di)=(1/f), but hten i realized i don't have the object distance, so what formula can i use?

 

[vaishakh]

There is a formula for magnification? Try if you remember.

 

[Hootenanny]

Try http://hyperphysics.phy-astr.gsu.edu/ this usually gives good information. I believe you assume that object distance, which in this case is the slide, is zero.