1. 9-5 Area of Circles Warm Up Problem of the Day Lesson Presentation
Course 2
Warm Up
Problem of the Day
Lesson Presentation

2. 9-5 Area of Circles Warm Up Simplify. 1. 82 2. 122 3. 6.22 64 4. 7.52
Course 2
9-5
Area of Circles
Warm Up
Simplify.
1. 82
2. 122 64
144
38.44
56.25

3. 9-5 Area of Circles Problem of the Day
Course 2
9-5
Area of Circles
Problem of the Day
A 16 in. pizza sells for $ A 10 in. pizza sells for $5.99. Which size gives you more pizza per penny? Explain.
The larger pizza; you get more than twice as much (64 in2 instead of 25 in2) for about twice the price.

4. Course 2
9-5
Area of Circles
Learn to find the area of circles.

5. 9-5 Area of Circles So the area of the parallelogram can be written as
Course 2
9-5
Area of Circles
A circle can be cut into equal-sized sectors and arranged to resemble a parallelogram. The height h of the parallelogram is equal to the radius r of the circle, and the base b of the parallelogram is equal to one-half the circumference C of the circle.
So the area of the parallelogram can be written as 1 2
A = bh, or A =
Cr. 1 2
Since C = 2r, A =
(2r)r = r2.

6. • 9-5 Area of Circles AREA OF A CIRCLE A = r2
Course 2
9-5
Area of Circles
AREA OF A CIRCLE
The area A of a circle is the product of  and the square of the circle’s radius r.
A = r2 • r

7. Additional Example 1A: Finding the Area of a Circle
Course 2
9-5
Area of Circles
Additional Example 1A: Finding the Area of a Circle
Find the area of the circle to the nearest tenth. Use 3.14 for .
A = r2
Use the formula.
7 cm
A  3.14 · 72
Substitute 7 for r.
A  3.14 · 49
Evaluate the power.
A 
Multiply.
The area of the circle is about cm2.
The order of operations calls for evaluating the exponents before multiplying.
Remember!

8. Additional Example 1B: Finding the Area of a Circle
Course 2
9-5
Area of Circles
Additional Example 1B: Finding the Area of a Circle
Find the area of the circle to the nearest tenth. Use 3.14 for .
A = r2
Use the formula.
18 ft
A  3.14 · 92
Substitute 9 for r.
A  3.14 · 81
Evaluate the power.
A 
Multiply.
The area of the circle is about ft2.

9. 9-5 Area of Circles Check It Out: Example 1A
Course 2
9-5
Area of Circles
Check It Out: Example 1A
Find the area of the circle to the nearest tenth. Use 3.14 for .
A = r2
Use the formula.
10 cm
A  3.14 · 102
Substitute 10 for r.
A  3.14 · 100
Evaluate the power.
A  314
Multiply.
The area of the circle is about 314 cm2.

10. 9-5 Area of Circles Check It Out: Example 1B
Course 2
9-5
Area of Circles
Check It Out: Example 1B
Find the area of the circle to the nearest tenth. Use 3.14 for .
A = r2
Use the formula.
12 ft
A  3.14 · 62
Substitute 6 for r.
A  3.14 · 36
Evaluate the power.
A 
Multiply.
The area of the circle is about ft2.

11. Additional Example 2: Application
Course 2
9-5
Area of Circles
Additional Example 2: Application
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? Use 22 7
for .
A =  r2
Use the formula for the area of a circle. 22 7
A 
·142
Substitute. Use 14 for r. 28 22 7
A 
196
Evaluate the power. 1
A  22 · 28
A  616
Multiply.
The area of the top that will cover the drain is about 616 in2.

12. 9-5 Area of Circles Check It Out: Example 2
Course 2
9-5
Area of Circles
Check It Out: Example 2
Albert was designing a cover for a spa. If the radius of the spa is 7 ft, what is the area of the cover that will be made? Use 22 7
for .
A =  r2
Use the formula for the area of a circle. 22 7
A 
·72
Substitute. Use 7 for r. 7 22 7
A  49
Evaluate the power. 1
A  22 · 7
A  154
Multiply.
The area of the top that will cover spa is about 154 ft2.

13. Additional Example 3: Application
Course 2
9-5
Area of Circles
Additional Example 3: Application
Find the area of the shaded region of the circle. Use 3.14 for  . Round your answer to the nearest tenth.
1.5 cm
The measurement of the radius of the circle is 1.5 cm.
Now find the area of the entire circle.
A = r2
Use the formula for the area of a circle.
A = 3.14 · 1.52
Substitute. Use 1.5 for r.
A  3.14 · 2.25
Evaluate the power.
A  7.07
Multiply.

14. Additional Example 3 Continued
Course 2
9-5
Area of Circles
Additional Example 3 Continued
Find the area of the shaded region of the circle. Use 3.14 for  . Round your answer to the nearest tenth.
Since of the circle is shaded, divide the area of the
circle by ÷ 2 =
1 2
The area of the shaded region of the circle is about 3.5 cm2.

15. 9-5 Area of Circles Check It Out: Example 3
Course 2
9-5
Area of Circles
Check It Out: Example 3
Find the area of the shaded region of the circle. Use 3.14 for  . Round your answer to the nearest tenth.
2.4 cm
The measurement of the radius of the circle is 2.4 cm.
Now find the area of the entire circle.
A = r2
Use the formula for the area of a circle.
A = 3.14 · 2.42
Substitute. Use 2.4 for r.
A  3.14 · 5.76
Evaluate the power.
A  18.09
Multiply.

16. Check It Out: Example 3 Continued
Course 2
9-5
Area of Circles
Check It Out: Example 3 Continued
Find the area of the shaded region of the circle. Use 3.14 for  . Round your answer to the nearest tenth.
Since of the circle is shaded, divide the area of the
circle by 4 and subtract the answer from the entire area ÷ 4 = – 4.52 =
3 4
The area of the shaded region of the circle is about cm2.

17. Insert Lesson Title Here
Course 2
9-5
Area of Circles
Insert Lesson Title Here
Lesson Quiz
Find the area of each circle, to the nearest tenth. Use 3.14 for . 1.
3. The bull’s-eye on a target has a diameter of 2 in. What is the area of the bull’s-eye to the nearest tenth? Use 3.14 for .
4. A round table cloth has a radius of 36 in. What is the area of the table cloth? Use 3.14 for . Round your answer to the nearest tenth. 2.
9 ft
8 ft • •
201.0 ft2
63.6 ft2
3.1 in2
4,069.4 in2