Why do satellites launched into low-earth orbits not remain there indefinitely

The only way to remain in orbit indefinitely is to make sure that no part of the orbit passes through any air molecules. There are too many of those in a low earth orbit. So a low earth orbit gradually loses energy when the satellite hits air molecules.

d

explanation:

v=uf/(u-f)

= 5.10/(5-10) convex lens has positive focal length

=50/-5

=-10 cm (formed at other side)