What is a circle? It is a curve on which all the
points are at the same distance from the origin of the circle.
This distance is the radius of the circle. The diameter
of a circle is two times the radius.
The circumference of a circle is always a constant multiple
of its diameter. This is a fundamental truth in the universe.
The Greek symbol π is used to represent this constant.
To five decimal places π is approximately 3.14159.
Thus, a circle with a radius 5 cm has a circumference
approximately: cm
The area of a circle is the radius squared × π, or
π r^{2}.
Thus, a circle with a radius 10 cm has an area
approximately:
cm^{2}