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# Circumference and Diameter - PowerPoint PPT Presentation

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 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
• 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.
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.

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

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
• Example 1: The radius of a circle is 2 inches. What is the diameter?
• d = 2 · r
• d = 2 · (2in)
• d = 4in
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
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
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
• 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

Created by Christopher Louck

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