Circumference is defined as the distance around a circle; the diameter is the straight-line distance across the circle that touches two points on its border and goes through its center. The ratio of a circle's circumference to its diameter is the constant pi. In other words, circumference/diameter = pi, or circumference = pi * diameter. Therefore, the length of the diameter of a circle is equivalent to its circumference divided by pi.

Similarly, the length of a circle's radius, (the distance from the circle's center to any point on its border), is equivalent to its circumference divided by (2*pi). The radius is half as long as the diameter.

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