hi sir
i am sara i want to ask a doubt
in a cone and cylender the curved sides are not counted in case of faces
why is it so ??

Hi, 
Face is considered in cubes, cuboids , etc which has flat face and we count only flat faces and not the circular faces. In other words circular faces are also called lateral face and they are totally different from flat faces. 
In simple words if a 3-dimension figure if you can put its face on table and if it is totally covered by surface of table then we count them.