Final answer:
The radius of a circle is used instead of the diameter when calculating circumference because it simplifies the calculations and avoids the need for dividing the diameter by 2. The radius is also used in other circle-related formulas.
Step-by-step explanation:
When calculating the circumference of a circle, it is generally more convenient to use the radius rather than the diameter. The formula for the circumference of a circle is C = 2πr, where r is the radius. Using the radius allows for simpler calculations and avoids the need for dividing the diameter by 2. Additionally, many other formulas related to circles, such as the area of a circle and the properties of circular orbits, rely on the use of the radius.
Learn more about radius of a circle