Finding Focal Length of Diverging Lens with Parallel Rays

[langenase]

Homework Statement

I'm trying to find the focal length of the diverging lens. The converging lens is 24cm behind a diverging lens. The focal length of the converging lens is 31cm. I know that parallel light strikes the diverging lens and then affter passing through the converging lens the light is again parallel.

The Attempt at a Solution

I know that the image for the diverging lens becomes the object for the converging lens. Does this mean that the distance between the diverging and converging lens is also the distance between the object and the converging lens?


If the rays are parallel when they enter the diverging lens then the distance between the object and the diverging lens is infinity. The distance between the image and the converging lens should also be infinity.
So for the converging lens:
1/f = 1/do + 1/infinity

I don't exactly know what to do with 'do'. I know its equal to the image of the diverging lens but I don't get the right answer when I substitute 1/fd - 1/infinity = 1/do in for 1/do in the converging equation.

I'd appreciate any suggestions!
Thank you so much..this is driving me crazy!

 

[andrevdh]

one over a very large number is a very small number so you can ignore the term 1/infinity

another point is that for the diverging lens the object is at infinity so the term should be 1/di

since the image is virtual and forms to the left of the diverging lens the term should actually be -1/di