1. 9.3
Area of Circles
How do you find the area of a circle?

2. ADDITIONAL EXAMPLE 1
A remodeling project calls for sanding a chair with a disk sander. The sanding disk used on the sander has a radius of 4.5 inches. Find the area of the disk. Use 3.14 for π.
about in2

3. 9.3 LESSON QUIZ
7.8.C
For 1–3, find the area of each circle. Use 3.14 for π. Round to the nearest tenth if necessary. 1.
a circle with radius 48 mm
7, mm2 2.
a circle with diameter 8.4 ft
55.39 ft2 3.
a circle with diameter 14 in.
in2 4.
The diameter of the face-off circle on an ice hockey rink is 30 feet. Find its area.
706.5 ft2 5.
A circle has a circumference of 16π yards. Find the area in terms of π.
64π yd2

4. 6.
The Garcia family bought 400 yards of fencing to enclose a circular wildflower garden. They will use all of the fencing. a.
What is the diameter of the garden?
about yd b.
Use your answer to part a to find the area enclosed by the fence?
about 12, yd2

5. Use the figure at the right to see how close you can estimate the area of a circle using squares.
The diameter of the circle is equal to the side length of the larger square in the figure. What is the area of the larger square if the diameter of the circle is 10 cm?
100 cm2

6. Use the figure at the right to see how close you can estimate the area of a circle using squares.B
The diameter of the circle is also equal to the diagonal of the smaller square shown in the figure. The side length of the smaller square is about 7.07 cm. What is the area of the smaller square?
about 50 cm2

7. Sample answer: 75 cm2; 75 cm2 is the average of 100 cm2 and 50 cm2.
Use the figure at the right to see how close you can estimate the area of a circle using squares. C
Estimate the area of the circle. Justify your reasoning.
Sample answer: 75 cm2; 75 cm2 is the average of 100 cm2 and 50 cm2. D
Find the actual area of the circle. Useπ 𝛑 = 3.14.
A = π(5)2 = 3.14(25) = 78.5; about 78.5 cm2

8. Sample answer: The estimate is close to the actual area.
Use the figure at the right to see how close you can estimate the area of a circle using squares. E
How does your estimated area of the circle compare to its actual area?
Sample answer: The estimate is close to the actual area.

9. How do you find the area of a circle?
Sample answer:
Use the formula A = 𝜋r2, where A is the area and r is the radius. Use 3.14 or for 𝜋.