A [?] is a shape with all points the same distance from the center. If you measure the distance around a circle and divide it by the distance across the circle through the center, you will always come close to a particular value, depending upon the accuracy
of your measurement. This value is approximately 3.14159265358979323846... We use the Greek letter Pi to represent this value. However, using computers, Pi has been calculated to over 1 trillion digits past the decimal point.

The distance around a circle is called the [?]. The distance across a circle through the center is called the diameter. Pi is the ratio of the circumference of a circle to the diameter. The [?] of a circle is the distance from the center of a circle to any point on the circle. If you place two radius end-to-end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is [?] as long as the radius.

Circumference, diameter and radius are measured in [?] units, such as inches and centimeters. A circle has many different radius and many different diameters, each passing through the center.