Circumference formula
A circle is a shape with all points the same distance from the center. It's called by the center. The circle to the right is named circle A since the center is at point A. If you calculate the distance around a circle and slice it by the distance across the circle through the center, you'll perennially come close to a particular value, depending upon the accuracy of your measurement. This value is approximately 3.141592653... We use expression named PI to represent this value. The expression PI goes on always. But using computers, mathematicians have been able to measure the value of PI to thousands of places.
The distance around a circle is called the circumference. The distance across a circle through the center is called the diameter, diameter divided from 2 is called R (radius). PI is the ratio of the circumference of a circle to the diameter. Thus, for any circle, if you divide the circumference by the diameter, you get a value close to PI. This relationship is expressed in the following formula: C/d= PI. ~3.14
where C is circumference and d is diameter. You could test this formula at home with a round dinner plate. If you calculate the circumference and the diameter of the plate and then divide C by d, your quotient must come close to PI. Other way to write this formula is: C = PI * d where * means multiply. This 2nd formula is commonly used in problems where the diameter is given and the circumference isn't known.
The radius R of a circle is the distance from the center of a circle to any point on the circle and R * 2 = d. Because the diameter of a circle is wto time as long as the radius. This relationship is expressed in the following formula: d = 2 * r, d where d is the diameter and r is circle the radius.
Circumference, diameter and radius are calculated in linear units, such as inches, meaters or centimeters. A circle has lot of different 
radius and lot of different diameters, each passing through the center. A real-life example of a radius is the spoke of a bicycle wheel. A 30 centimeters pizza is an example of a diameter: when one makes the 1st cut to divide a round pizza pie in half, this cut is the diameter of the pizza. So a 30 centimeters pizza has a 30 centimeters diameter. Let's look at some examples of finding the circumference of a circle. In these examples, we will use PI = 3.14 to simplify our calculations.
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