A convex lens of focal length `f_(1)` is kept in contact with a concave lens of focal length `f_(2)`. Find the focal length of the combination.

A convex lens of focal length f is put in contact with a concave lens of the same focal length. The equivalent focal length of the combination is :

A converging lens of focal length 40 cm is kept in contact with a diverging lens of focal length 30 cm. Find the focal length of the combination.

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:

A convex lens of focal length cc"f" is put in contact with a concave lens of the same focal length. The equivalent focal length of the combination is :

A thin convex lens of focal length 20 cm is kept in contact with a thin concave lens of focal length 15 cm. Find the focal length and the nature of the combination.

A convex lens of focal length 10.0 cm is placed in contact with a convex lens of 15.0 cm focal length. What is the focal length of the combination ?

A convex lens of focal length 'f' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is