A sphere is a body bounded by a surface whose every point is equidistant (i.e. the same distance) from a fixed point, called the centre. For example, a shot (a heavy iron ball) is a solid sphere and a tennis ball is a hollow sphere.

One-half of a sphere is called a hemisphere.

Archimedes discovered that a cylinder that circumscribes a sphere, as shown in the following diagram, has a curved surface area equal to the surface area, S, of the sphere.

Example 33

Solution:

Note:

We often use a circle to represent a sphere.  The three-dimensional figure of a sphere is drawn by a circle and an ellipse as illustrated above.

Key Terms

sphere, equidistant, hemisphere, surface area