1. Bell Work Find the circumference of the circles.
52mm
C = πd
C = 2πr
C = (3.14)(12)
C = (2)(3.14)(52)
C = ft
C = mm

2. Circles and Area

3. How in the world would you find the area of a circle?

4. Remember! Area is always measured in square units.

5. There are 8 squares in the rectangle.
Let’s look at a rectangle. Area = (Length)(Width) (Hint: you’re counting the number of squares inside of the rectangle)
A = L x W
A = (4)(2)
A = 8 1 2 3 4 2 5 6 7 8
There are 8 squares in the rectangle. 4

6. Estimate the number of square units inside the circle.
Now consider a circle.
Estimate the number of square units inside the circle.
There are about 12 squares plus the 4 parts that are approximately of a square each. 1 2 3 4 5 6 7 8 9 10 11 12
There are about 13 square units inside this circle.

7. So the area of the circle is: Area =πr2
This is just an ESTIMATE though.
How can we find the exact area?
Area of a Circle
So the area of the circle is:
Area =πr2

8. Area = πr2 Area = π(14)2 Area = (3.14)(14)2 Area = (3.14)(196)
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?
Area = πr2
Area = π(14)2
Area = (3.14)(14)2
Area = (3.14)(196)
Area = in²
Remember: We use 3.14 for π. Always round to the nearest hundredth.

9. A circular flower bed in Kay’s backyard has a diameter of 9 feet
A circular flower bed in Kay’s backyard has a diameter of 9 feet. What is the area of the flower bed?
Area = πr2
Area = π(4.5)2
Area = (3.14)(4.5)2
Area = (3.14)(20.25)
Area = ft²
NOTICE: We have the diameter but we need to find the radius.
Remember: We use 3.14 for π. Always round to the nearest hundredth.

10. How would you find the shaded area?
Find the area of the square and subtract the area of the circle.
ASQUARE = (2)(2)
ASQUARE = 4 ft²
ACIRCLE = π(1)²
ACIRCLE = 3.14 ft²
ASHADED =
ASHADED = 0.86 ft²

11. What if we wanted to find the radius and diameter of a circle given the area?
Use π = 3.14 to find the radius of this circle.
A = πr²
How can we rearrange this to make the radius the subject of the formula?
A = 100 cm ?
r² = A π
r =
r = 5.64 cm
We can also find the diameter since
d = 2r
Always round your answer to the nearest hundredth.
d = cm

12. Example Area = πr2 1,386 = (3.14)r2 r = r = 21.01 in
A tablecloth for a round table has an area of 1,386 in². What is the approximate radius of the tablecloth?
Area = πr2
1,386 = (3.14)r2
r =
r = in

13. Practice: Area Worksheet