1. Warm Up
Problem of the Day
Lesson Presentation
Lesson Quizzes

2. Warm Up
Simplify.
1. 82
2. 122 64
144
38.44
56.25

3. 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. Learn to find the area of circles.

5. So the area of the parallelogram can be written as
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.

7. 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
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. Find the area of the circle to the nearest tenth. Use 3.14 for .
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. Find the area of the circle to the nearest tenth. Use 3.14 for .
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
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. The area of the top that will cover spa is about 154 ft2.
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
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
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. 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
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.