Transcript Circumference and Area of Circles

Circumference and Area of Circles
Parts of a Circle
• Radius-Straight line from the middle to the
outside of a circle
radius
• Diameter-a line that cuts the circle in half
from end to end
diameter
Circumference
• Circumference is the distance around the
circle. Like the perimeter.
Pi
• Pie is a special number. It is the ratio of
the circumference and the diameter

• Pi goes on forever!!!!!!!!!!!
• We approximate it using 3.14
  3.14
Finding Circumference
C  d
Finding Circumference
6in
C  d
C   (6)
6 · 3.14 = 18.84 in.
6𝛑 in. would be the exact answer, but we are
going to estimate using 𝛑 = 3.14
3in
12in
1in
What if you have the radius?
The radius is half of the diameter
4
C  8
2
3
How Would You Find the Diameter if
You Only Had the Circumference?
• It’s easy!
• When given circumference, to find
diameter just divide by pi.
Circumference = 𝛑d
21.98 = 3.14d
C = 21.98 cm
d
r
Circumference = 𝛑d
C = 37.68 m
d
r
Circumference = 𝛑d
C = 47.10 m
d
r
I thought penguins were smart?
Area of a Circle
Area of a Circle
The Formula
Area   r
2
Example
A r
2
A   (6)
6in
2
A   36
3.14 · 36 = 113.04 in²
Try
4in
Try
5in
What if you are given the diameter?
• How can you find the radius if you only
have the diameter?
• The radius is half of the diameter
• Diameter = 8 then radius =4
Radius = 3
A r
2
A   (3)
A  9
2
Example
6in
Try
4in
14in
2in
Finding Radius and Diameter
from Area
• Just like you could find radius and
diameter from circumference, you can find
the radius and diameter from area by filling
in known values and solving.
Example
Area = 𝛑r²
Area = 78.50 cm²
Radius =
Diameter =
Area = 314.00 cm²
Radius =
Diameter =
Area = 200.96 cm²
Radius =
Diameter =
Closure
• What is the formula for finding the area of
a circle?
Closure
• How can you find the radius if you only
have the diameter?
Closure
• What is the diameter?
Closure
• What is the radius?
Closure
• What is circumference?
Closure
• What is the relationship between the
radius and the diameter?