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 Area of a Circle Unit 2 > Lesson 3 of 6

 The distance around a circle is called its circumference. The distance across a circle through its center is called its diameter. We use the Greek letter (pronounced Pi) to represent the ratio of the circumference of a circle to the diameter. In the last lesson, we learned that the formula for circumference of a circle is: . For simplicity, we use = 3.14. We know from the last lesson that the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula: .
 The area of a circle is the number of square units inside that circle. If each square in the circle to the left has an area of 1 cm2, you could count the total number of squares to get the area of this circle. Thus, if there were a total of 28.26 squares, the area of this circle would be 28.26 cm2 However, it is easier to use one of the following formulas: or where is the area, and is the radius. Let's look at some examples involving the area of a circle. In each of the three examples below, we will use = 3.14 in our calculations.

 Example 1: The radius of a circle is 3 inches. What is the area? Solution: = 3.14 · (3 in) · (3 in) = 3.14 · (9 in) = 28.26 in Example 2: The diameter of a circle is 8 centimeters. What is the area? Solution: 8 cm = 2 · 8 cm ÷ 2 = = 4 cm = 3.14 · (4 cm) · (4 cm) = 50.24 cm Example 3: The area of a circle is 78.5 square meters. What is the radius? Solution: 78.5 m = 3.14 · · 78.5 m ÷ 3.14 = · 25 m = · = 5 m

 Summary: Given the radius or diameter of a circle, we can find its area. We can also find the radius (and diameter) of a circle given its area. The formulas for the diameter and area of a circle are listed below: or

### Exercises

 Directions: Read each question below. Click once in an ANSWER BOX and type in your answer; then click ENTER. After you click ENTER, a message will appear in the RESULTS BOX to indicate whether your answer is correct or incorrect. To start over, click CLEAR. Use = 3.14 to calculate your answers.

 1 The radius of a circle is 9 centimeters. What is the area? ANSWER BOX:   = cm RESULTS BOX:

 2 The diameter of a circle is 12 inches. What is the area? ANSWER BOX:   = in RESULTS BOX:

 3 The radius of a circular rug is 4 feet. What is the area? ANSWER BOX:   = ft RESULTS BOX:

 4 The area of a coin is 3.14 square centimeters. What is the radius? ANSWER BOX:   = cm RESULTS BOX:

 5 The diameter of a bicycle wheel is 20 inches. What is the area of the wheel? ANSWER BOX:   = in RESULTS BOX:

This lesson is by Gisele Glosser. You can find me on Google.

 Circumference and Area Lessons Geometry and the Circle Circumference of a Circle Area of a Circle Practice Exercises Challenge Exercises Solutions

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