A convex lens of focal length f1 is kept in contact with a concave lens of focal length f2. Find the focal length of the combination.

Convex lens has positive focal length whereas the concave lens has negative focal length.
∴fconvex=f1 and fconcave=-f2
Focal length of the combination of lens
1/F = 1/fconvex + 1/fconcave
∴1/F = 1/f1 + 1/-f2 ⇒ F = f1f2/(f2-f1)