How do you find the focal distance of a divergent lens?

I really need to copy the diagram, but you can use a formula as well. Normally you take the lens and measure the distance a point is focused from it, the analogue method.
I-mage is formed with an extra converging lens, of O-bject placed in front of it. Then on the other side you place the divergent lens and compare the distance when it's put in with the distance from the concave lens alone.
Then you use a formula basically taking the two distances and multiplying them up, but the book lost me at that point. Alternatively you get a lens meter and press the points on the lens which will read it directly.

Next we will find the focal length of a diverging lens. We will do this by two methods.
In the first method we will use a converging lens ahead of a diverging lens to converge the rays before they
reach the diverging lens thus using the image of the converging lens as the object for the diverging lens.
Place the arrow at one end of the bench and position the converging lens a distance of 4f from the object (as
you’ve done earlier). Calculate the image location (di1) using the thin lens equation. Now place the
diverging lens 8 cm away from the converging lens. Calculate the object distance of lens 2 (do2) by
subtracting the separation distance of the lenses from di1. Next move the white screen until the image is in
focus and record the distance between the screen and lens 2 (di2). Calculate the focal length of the
diverging lens (F2) using the thin lens equation.


We can also calculate the focal length of the diverging lens by placing the converging lens next to
the diverging lens and measuring the combined focal length by focusing on a distant object. If two thin
lenses are placed close together the effective focal length fc of the combination is given by the expression

at this point you will have to go to the site to see the calculation

The student may easily derive this result from the thin lens equation by noting that the object distance for
the second lens is, to the degree of approximation of the thin lens equation, numerically equal to the image
https://en.wikipedia.org/wiki/Focal_length

Now class, wasn’t that easy!!! LOL!!!