Two thin lenses of power + 6 D and - 2D are in contact. What is focal length of the combination?

Two thin lenses of pwer +5D and -3D are in contact. What is the focal length of the combnation?

Two lenses of power 15.5 D and -5.5 D are placed in contact.What is teh focal length of the combbination?

Two lenses f powers-1.5 and +2.75 D are kept in contact.Find the focal length of the combination.

Two lenses of powers+15D and -5D are in contact with each other. What is the focal length of combination?

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 lenses of focal lenths + 10 cm and - 5 cm are kept in contact.What is the focal length and

Two lenses having powers + 6 D and - 4D are placed in contact.The power of the combintin is

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