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Pyramid and Spheres
Pyramids are polyhedra because they are made up of plane faces. Spheres are not polyhedra because they are curved.

Pyramids
If a line that is fixed at a point, called the vertex, is moved around the perimeter of a polygon, then a pyramid is formed.

The following solid is pyramid.



Pyramids are named after their base. So, the pyramid shown above is an example of a rectangular pyramid.

Some other pyramids are shown below.



A pyramid is said to be right when its vertex is directly above the centre of the base. The cross-sections parallel to the base are the same shape but have different sizes.




The net of the above pyramid consists of:

a rectangular base; and
four triangular faces


Note:
The cross-sections parallel to the base have the same shape as the base but different sizes.
The net of a pyramid consists of a base polygon and a number of triangular faces.
A regular pyramid has a base that is a regular polygon and has faces which are isosceles triangles.

Spheres
If a semicircle is revolved about its diameter, then a sphere is formed.

The following solid is a sphere.



The cross-sections...