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 Circumference of a Circle Part II Unit 2 > Lesson 2 of 6

 Circumference, diameter and radii are measured in linear units, such as inches and centimeters. A circle has many different radii and many different diameters, each passing through the center. A real-life example of a radius is the spoke of a bicycle wheel. A 9-inch pizza is an example of a diameter: when one makes the first cut to slice a round pizza pie in half, this cut is the diameter of the pizza. So a 9-inch pizza has a 9-inch diameter. Let's look at some examples of finding the circumference of a circle. In these examples, we will use = 3.14 to simplify our calculations.

 Example 1: The radius of a circle is 2 inches. What is the diameter? Solution: = 2 · (2 in) = 4 in Example 2: The diameter of a circle is 3 centimeters. What is the circumference? Solution: = 3.14 · (3 cm) = 9.42 cm Example 3: The radius of a circle is 2 inches. What is the circumference? Solution: = 2 · (2 in) = 4 in = 3.14 · (4 in) = 12.56 in Example 4: The circumference of a circle is 15.7 centimeters. What is the diameter? Solution: 15.7 cm = 3.14 · 15.7 cm ÷ 3.14 = = 15.7 cm ÷ 3.14 = 5 cm

 Summary: The number is the ratio of the circumference of a circle to the diameter. The value of is approximately 3.14159265358979323846...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 formulas for diameter and circumference of a circle are listed below. We round to 3.14 in order to simplify our calculations.