**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 of a sphere are circles. The cross-sections that
pass through the centre of the sphere are called **great circles** whilst
other cross-sections are called **small circles**.
A hemisphere is a half sphere.
Key Terms
pyramid, vertex of a pyramid, base of a pyramid, rectangular pyramid, triangular pyramid, hexagonal
pyramid, octagonal pyramid, right
pyramid, net of a pyramid, regular
pyramid, sphere, centre of a
sphere, great circles of a sphere, small
circles of a sphere, hemisphere |