Two thin lenses of focal lengths `f_(1)` and `f_(2)` are in contact. The focal length of this combination is

Two thin lenses of focal length f_(1) and f_(2) are in contact and coaxial. The power of the combination is

Two thin lenses of focal length f_(1) and f_(2) are in contact and coaxial. The power of the combination is

Two thin lenses of focal length f_1 and f_2 are placed in contant .The focal length of the composite lens will be

Two thin convex lenses of focal lengths f_(1) and f_(2) are in contact and coaxial. The power of the combination is

Two thin convex lenses of focal length f_1 and f_2 are kept in contact. The power of the combination is