8 – 3 Circumference and Area
of a Circle
Mr. Dixon
Math 8
Vocabulary
Circle: consists of all
points in a plane that are
the same distance from a
fixed point called the
center.
Center: the point inside
the circle that is the same
distance from all points on
the circle.
Circumference: the distance around the
circle. Circumference is measure in
units.
Circles


Diameter: the distance
from the circle through
the center.
Radius: distance from
the center to any point
on the circle.
Radius is half of the
diameter.
Circumference of A Circle
Symbols:
r = radius
d = diameter
π = (3.14)
Formulas: C = π d or C = 2 π r
Example #1; finding the circumference
Formulas: C = π d or C = 2 π r
Example #1: A circular label has a diameter of 3.33
inches. Approximate the distance around the label
to the nearest tenth of an inch.
C = πd
3.33 in.
C = (3.14)(3.33)
C = 10.4562
C = 10.5 inches
Example #2; finding the radius of circle
Formulas: C = π d or C = 2 π r
Example #2: The circumference of a circle is 56 ft. Find the
radius of the circle to the nearest foot.
C = 2πr
56 ft = (2)(3.14)(r)
56 = 6.28r
r = 8.92 feet = 9 ft.
Formula for Area
A  r2
 = 3.14
r2 = r • r
Example 3: Area of Circle
Find the Area of the circle. Round to the nearest tenth.
1) Find the radius
r=5m
2) Find the area
Write the formula
Substitute what you know
Simplify
A  r2 A  (3.14)(5)2
A  78.5
A  78.5 meters2
d = 10 m
10 m
Example 4: Radius of a circle
The area of a circle is 39.25 square yards. Find the radius of the
circle to the nearest tenth of a yard.
A  r2
39.25  (3.14)r2
12.5 = r2
√12.5 = r
3.5 = r
r  3.5 yards
Checkpoint; #1
#1: Find the circumference of the circle.
Round to the nearest tenth.
14 cm
C = 2πr
C = 2(3.14)(14)
C = (6.28)(14)
C = 87.92
C = 87.9 cm
Extra Practice
Find the area of the circle.
A  r2
5.3 m
A  (3.14)(5.3)2
A  88.2026
A  88.2 m2
ASSIGNMENT
Pg. 406, (1-22)all
Show all work on a separate sheet of paper.
Calculators may be used.