Question

a convex lens of power + 6D is placed in contact with the concave lens of power minus 4D what is the nature and the focal length of combination

Answer 1

P=p1+p2

So

P1=6D

P2=-4D

P=p1+p2

P=2D

It means it is a concave lens

Now

F=1/p

F=1/2

F= .5