When two thin lenses are kept in contanct, the focal length of the combination is

Two thin lenses of power 2D and -4D are kept in contact. What is the focal length of the combination in cm? (write the answer only)

Two lenses of power +2D and -5D are kept in contact. The focal length of the combination is

Two lenses of powers + 1.5 D and - 1 D are kept in contact. The focal length of the combination is

Two lenses of local lengths 0.20m and 0.30m are kept in contact. Find the focal length of the combination. Calculate power of two and combination.

" two thin lenses of equal focal length but of opposite nature are pair in contact,the resultant focal length of the combination will be "

Two thin lenses of power + 3 D and - 1 D are held in contact with each other. Focal length of the combination is :