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Circumference and Diameter. 1/31/2006. Circumference. A circle is a shape with all points the same distance from the center. It is named by the center. The circle to the left is called circle A since the center is at point A. Circumference.

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circumference
Circumference
  • A circle is a shape with all points the same distance from the center. It is named by the center.
  • The circle to the left is called circle A since the center is at point A.
circumference1
Circumference
  • 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 Pi to represent this value.
circumference2
Circumference

The distance around a circle is called the circumference. 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. Thus, for any circle, if you divide the circumference by the diameter, you get a value close to Pi.

circumference3
Circumference

You can test this formula at home with a round dinner plate. If you measure the circumference and the diameter of the plate and then divide C by d, your quotient should come close to Pi. Another way to write this formula is:

C = Pi· d where “·” means multiply. This second formula is commonly used in problems where the diameter is given and the circumference is not known.

radius
Radius

The radius of a circle is the distance from the center of a circle to any point on the circle. If you place two radii end-to-end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula:

d = 2 ·r where d is the diameter and r is the radius.

examples
Examples
  • Example 1: The radius of a circle is 2 inches. What is the diameter?
  • d = 2 · r
  • d = 2 · (2in)
  • d = 4in
examples1
Examples
  • Example 2: The diameter of a circle is 3 centimeters. What is the circumference?
  • C = Pi · d
  • C = 3.14 · (3cm)
  • C = 9.42cm
examples2
Examples
  • Example 3: The radius of a circle is 2 inches. What is the circumference?
  • d = 2 · r
  • d = 2 · (2in)
  • d = 4in
  • C = Pi · d
  • C = 3.14 · (4in)
  • C = 12.56in
examples3
Examples
  • Example 4: The circumference of a circle is 15.7 centimeters. What is the diameter?
  • C = Pi · d
  • (15.7cm) = 3.14 · d
  • 15.7 cm ÷ 3.14 = d
  • d = 5cm
summary
Summary
  • The number Pi is the ratio of the circumference of a circle to the diameter.
  • The value of Pi is approximately 3.14.
  • The diameter of a circle is twice the radius. Given the diameter or radius of a circle, we can find the circumference.
  • We can also find the diameter (and radius) of a circle given the circumference.
  • The formula for diameter is d = 2 · r
  • The formula for circumference is C = Pi · d
slide12

Created by Christopher Louck

  • http://www.kent.k12.wa.us/staff/ChristopherLouck/math.htm
  • Used by permission
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