It is called a frustum.
it a cone cut in half from the top
You simply calculate it like a cone, but the height of the cone is the height to the top of the FILLED part, not all the way. Half-filled is not enough information . . . there can be "half filled" meaning half the height of the cone, but can also be "half filled" meaning half the volume of the cone.
Half of its diameter
nappe :)
pls tell me how to find development for cone which is half cutted
I don't think there is a special name for that.
A cylinder and a cone have infinitely many planes of symmetry because of the circular face. However, a cylinder can also be cut in half lengthwise (imaging cutting a soda can in half), while a cone cannot (imagine cutting a ice cream cone in half). Therefore, a cylinder has one more plane of symmetry than a cone.
It is called a frustrum
two
Nocturnal
Oast
a cone