Hello aspirant,

Hope you are doing well!!

Let Focal length F is the combination of both lenses. The formula that we use is :-

F = (uv) / (u+v)

Let the focal length of Convex lens is f1 .and concave lens is f2 We can determine the focal length of concave lens by this formula :-

f2 = (f1F) / (f1 - F)

I hope this information will help you.

Feel free to ask any query.

All the best