A lens of power +3.5D is placed in contact with a lens of power -2.5 D. The combination will behave like

A lens of power 6 D is put in contact with a lens of power -4 D. The combination will behave like a

A lens of power +3.5 diopters is placed in contact with a lens of power -2.5 diopter. The combination will behave like :

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 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 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 ?