WHAT IS A CIRCLE
Mathematically, a circle is a closed curve representing the set of points some fixed distance from a given point called the center. In the circle below the center point is labelled "O".
Radius: We call radius the distance between the center of a circle and any point along the circle. It is represented by the letter r.
Diameter: The diameter of the circle is the distance between two points along the circle measured through the center. It is represented by the letter d.
There exists an interesting relationship between the diameter and the radius of a circle. The diameter is two times the radius. Another way to say it is that the radius is half the diameter. So if you know the radius of a circle you can find the diameter by doubling it and vice versa. That is
d = 2r or r = d ÷ 2
PERIMETER OF A CIRCLE
The perimeter of a circle is called circumference. It is the distance around the circle. For all circles, the ratio of the circumference to the diameter, the distance around the circle to the distance across, is the same number. The value of this ratio has been given the label π, the Greek letter “pi.” The value of π is approximately 3.14
Ratio = circumference ÷ diameter = 3.14159 = π