2. Area of a Circle

3. How in the world would you find the area of a circle?

4. Remember! Area is always measured in square units.

5. Let’s look at a rectangle. Area = (Length)(Width) (Hint: you’re counting the number of squares inside of the rectangle) 2 4 A = L x W A = (4)(2) A = 8 1234 5678 There are 8 squares in the rectangle.

6. Now consider a circle. Estimate the number of square units inside the circle. There are about 13 square units inside this circle. There are about 12 squares plus the 4 parts that are approximately of a square each. 12 345 67 8 910 1112

7. This is just an ESTIMATE though. How can we find the exact area? Area of a Circle So the area of the circle is: Area = π r 2

8. But where does this formula come from? Let’s watch a video to find out. http://www.youtube.com/watch?v=YokKp3p wVFc

9. Park employees are fitting a top over a circular drain in the park. If the radius of the drain is 14 inches, what is the area of the top that will cover the drain? Area = π r 2 Area = π (14) 2 Area = (3.14)(14) 2 Area =(3.14)(196) Area = 615.44 in² Remember: We use 3.14 for π. Always round to the nearest hundredth.

10. A circular flower bed in Kay’s backyard has a diameter of 9 feet. What is the area of the flower bed? Area = π r 2 Area = π (4.5) 2 Area =(3.14)(4.5) 2 Area = (3.14)(20.25) Area = 63.59 ft² NOTICE: We have the diameter but we need to find the radius. Remember: We use 3.14 for π. Always round to the nearest hundredth.

11. The central performance area at a circus is a circular ring having a diameter of 42 ft. Half of the ring will be used for the clown show and half will be used for the unicycle show. How much area does each group get to perform in? A = 1384.74 ÷ 2 = 692.37 Find the radius: 42/2 = 21 ft

12. Find the area of the square and subtract the area of the circle. How would you find the shaded area? A SQUARE = (2)(2) A SQUARE = 4 ft² A CIRCLE = π(1)² A CIRCLE = 3.14 ft² A SHADED = 4 - 3.14 A SHADED = 0.86 ft²

13. Practice: Area Worksheet