1. Stand Quietly

2. Lesson 8.3 Areas of Circles
Day 1
Lesson 8.3 Areas of Circles
Students will be able to use inductive reasoning to understand the formula for the area of a circle as evidenced by the Circle Investigation activity.

3. Warm-Up #5 (9/12/2016)
Find the perimeter of a semicircle with a diameter of 2 meters. HINT: need to find the arc plus the bottom line).
A simple impact crater on the moon has a diameter of 15 km. A complex impact crater has a radius of 30 km. How much greater is the circumference of the complex impact crater than the simple impact crater?

4. Homework (9/12/16) Textbook Big Idea Page 336 #3-5 ALL, 22-25 ALL
REMEMBER TO BRING YOUR WORK BOOK TO SCHOOL EVERYDAY

5. Homework Solutions 1. 18 in 2. 6 in 3. 62.8 cm 4. 87.92 in 5. 25.12 ft
–47.1 = km

6. Exploration
Resource:

7. 1. Can you identify the new shape that you have created?
2. What is the formula for the area of the new shape?
3. What is the approximate height and base of the new shape? 
4. Find the area of the new shape? What can you conclude?
5. Write a formula for the area of a circle.

8. Lesson 8.3 Areas of Circles
Day 2
Lesson 8.3 Areas of Circles
Students will be able to find the area of a circle as evidenced by the practice examples.

9. Warm-Up #6 (9/13/2016)
Find the circumference of a circle with the radius of 10 in. =
What information do you need to know in order to find the area of a circle?

10. Homework (9/13/16) Textbook Big Ideas Page 336 #1, 7-19 ODD
REMEMBER TO BRING YOUR WORK BOOK TO SCHOOL EVERYDAY

11. Homework Solutions
3. About 𝑚𝑚 2 4. About 616 𝑐𝑚 2 5. About 314 𝑖𝑛 2
22. 44
23. 53
24. 74
25. A

12. Formula for the area of a circle
Area of a circle = πr2
radius
πrr

13. Use π = 3.14 to find the area of the following circles:
2 cm
10 m
A = πr2
A = πr2
= (3.14)(22)
= 3.14 × 52
= cm2
= 78.5 m2
A = πr2
23 mm
78 cm
A = πr2
= 3.14 × 392
Explain that rather than use the formula on the previous slide, it is usually easier to halve the diameter mentally to give the radius, before substituting it into the formula.
The most common error is to forget to half the diameter to find the radius.
= 3.14 × 232
= cm2
= mm2

14. Example 1
A circular sprinkler sprays water with a radius of 11 ft. How much area can the sprinkler cover?
Answer:
A = r2
A = 3.14(11)2
A = 3.14(121)
A = ft2

15. What is the area of a circle with a diameter of 24 yds?
Example 2
What is the area of a circle with a diameter of 24 yds?
Answer:
A = r2
A = 3.14(12)2
A = 3.14(144)
A = yd2

16. What is the radius of a circle whose area is 254.34 mm2?
Example 3
Answer: 9
A = r2
= r2
= 3.14r2
81 = r2
9 mm = r

17. A circular pool has an area of 153.86 ft2. What is its diameter?
Example 4
Answer: 14
A = r2
= r2
49 = r2
7 = r
14 ft = d