a convex lens of power `+6` dioptre is placed in contact with a concave lens of power`-4` dioptre. What will be the nature and focal length of this combination?

A lens of power +2 dioptres is placed in contact with a lens of power -1 dioptre. The combination will behave like

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. What is the nature and focal length of the combination ?

A convex lens of power +5 D is placed in contact with a concave lens of power -4 D. What is the power of combination and nature of lens combination ?

a convex lens of power 3.5D is placed in contact with a concave lens of power -1.5D. Find the effective focal length of the combination.

A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 10 cm. What is the focal length and power of the combination ?

A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the: a) power of this combination of lenses? b) focal length of this combination of lenses?