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

2. Warm Up
Evaluate.
1. 6  42
 23
Simplify each expression.
n – 4
4. 5(n + 1) 96
44.8
n + 2
5n + 5

3. Problem of the Day
What is the area of a right triangle with legs measuring 31 in. and 21 in.?
325.5 in2 3

4. Learn to find the area of a circle.

6. Additional Example 1A: Estimating the Area of a Circle
Estimate the area of the circle. Use 3 to approximate .
Write the formula for area.
A = r2
Replace  with 3 and r with 20.
A ≈ 3 • 202
Use the order of operations.
A ≈ 3 • 400
A ≈ 1,200 m2
Multiply.

7. Additional Example 1B: Estimating the Area of a Circle
Estimate the area of the circle. Use 3 to approximate .
A = r2
Write the formula for area.
r = d ÷ 2
The length of the radius is half the length of the diameter.
r = 28 ÷ 2
r = 14
Divide.
A ≈ 3 • 142
Replace  with 3 and r with 14.
A ≈ 3 • 196
Use the order of operations.
A ≈ 588 m2
Multiply.

8. Estimate the area of the circle. Use 3 to approximate .
Check It Out: Example 1A
Estimate the area of the circle. Use 3 to approximate .
Write the formula for area.
24.8 m
A = r2
Replace  with 3 and r with 25.
A ≈ 3 • 252
Use the order of operations.
A ≈ 3 • 625
A ≈ 1,875 m2
Multiply.

9. Estimate the area of the circle. Use 3 to approximate .
Check It Out: Example 1B
Estimate the area of the circle. Use 3 to approximate .
A = r2
Write the formula for area.
20 m
r = d ÷ 2
The length of the radius is half the length of the diameter.
r = 20 ÷ 2
r = 10
Divide.
A ≈ 3 • 102
Replace  with 3 and r with 10.
A ≈ 3 • 100
Use the order of operations.
A ≈ 300 m2
Multiply.

10. Additional Example 2A: Using the Formula for the Area of a Circle
Find the area of the circle. Use for . 22 7
A = r2
Write the formula to find the area.
r = d ÷ 2
The length of the diameter is twice the length of the radius.
r = 8 ÷ 2 = 4
A  • (4)2 22 7
Replace  with and r with 4. 22 7 __
A  • 16  22 7 __
352 7
Use the order of operations.
2 7
A  50 ft2
Divide.

11. Additional Example 2B: Using the Formula for the Area of a Circle
Find the area of the circle. Use for . 22 7
A = r2
Write the formula to find the area.
A  • 72 22 7
Replace  with and r with 7. 22 7 __
A  • 49 22 7 __ 7
Use the GCF to simplify. 1
A  154 cm2
Multiply.

12. Find the area of the circle. Use for . 22 7
Check It Out: Example 2A
Find the area of the circle. Use for . 22 7
28 cm
A = r2
Write the formula to find the area.
r = d ÷ 2
The length of the diameter is twice the length of the radius.
r = 28 ÷ 2 = 14
A  • 142 22 7
Replace  with and r with 14. 22 7 __
A  • 196 22 7 __ 28
Use the GCF to simplify. 1
A  616 cm2
Multiply.

13. Find the area of the circle. Use for . 22 7
Check It Out: Example 2B
Find the area of the circle. Use for . 22 7
7 cm
A = r2
Write the formula to find the area.
A  • 72 22 7
Replace  with and r with 7. 22 7 __
A  • 49 22 7 __ 7
Use the GCF to simplify. 1
A  154 cm2
Multiply.

14. Additional Example 3: Application
A drum head has a radius of 18 in. Find the area of the material covering the drum head. use 3.14 for .
A = r2
Write the formula for area.
18 in.
A  3.14 • 182
Replace  with 3.14 and r with 18.
A  3.14 • 324
Use the order of operations.
A  1, in.2
Multiply.

15. Additional Example 3 Continued
A drum head has a radius of 18 in. Find the area of the material covering the drum head. use 3.14 for .
Check
18 in.
Use 3 as an estimate for. The area, r2, is approximately
3 • 182 = 3 • 324 = 972, so the answer is reasonable.

16. A cymbal has a diameter of 18 in. Find its area. Use 3.14 for .
Check It Out: Example 3
A cymbal has a diameter of 18 in. Find its area. Use 3.14 for .
A = r2
Write the formula for area.
r = d ÷ 2
The length of the radius is half the length of the diameter.
18 m
r = 18 ÷ 2
r = 9
Divide.
A  3.14 • 92
Replace  with 3.14 and r with 9.
A  3.14 • 81
Use the order of operations.
A  m2
Multiply.

17. Check It Out: Example 3 Continued
A drum head has a radius of 18 in. Find its area. Use 3.14 for .
Check
18 m
Use 3 as an estimate for. The area, r2, is approximately
3 • 92 = 3 • 81 = 243, so the answer is reasonable.

18. Lesson Quizzes
Standard Lesson Quiz
Lesson Quiz for Student Response Systems

19. Estimate the area of each circle. 1. 2.
Lesson Quiz: Part I
Estimate the area of each circle.
1,200 yd2
27 km2 19

20. Find the area of each circle. Use for . 22 3. 4. 7
Lesson Quiz: Part II
Find the area of each circle. Use for . 22 7
1.54 m2
4.53 cm2
5. A coater has a diameter of 6 inches. Find the area of the largest cup the coaster can hold. Use 3.14 for .
28.26 in2 20

21. Lesson Quiz for Student Response Systems
1. Estimate the area of the circle. Use 3 for .
A. 48 km2
B. 46 km2
C. 44 km2
D. 42 km2
4 km

22. Lesson Quiz for Student Response Systems
2. Estimate the area of the circle. Use 3 for .
A. 772 yd2
B. 872 yd2
C. 972 yd2
D. 992 yd2
36 yd

23. Lesson Quiz for Student Response Systems
3. Estimate the area of the circle. Use 3 for .
A cm2
B. 21 cm2
C cm2
D cm2
3.5 cm

24. Lesson Quiz for Student Response Systems
4. A circular table has a diameter of 8 feet. Identify the area of the table cloth needed to cover the table. Use 3.14 for .
A ft2
B ft2
C ft2
D ft2