8.7 Area of Circles
Vocabulary
Circle: set of all points in a plane that are
the same distance from the center
Radius: distance from center to a point
on the circle
Diameter: distance across circle through
the center.
Circumference: distance around circle.
Circumference
Circumference: length around circle
Like perimeter
Circumference = 2 x π x (radius)
C = 2πr
radius
Find the circumference
C = 2 πr
5 in.
C = 2 π(11)
C = 22 π mi
C = 69.08 mi
C = 2πr
C = 2 π(5)
C = 10 π in.
C = 31.4 in.
11 mi
Try it at your desk!
P. 453 #1 - 3
Answers:
1. 38 cm
2. 57 ft
3. 50 in
Area
Area = π x (radius)2
A = πr2
7cm
A = πr2
A = π(7)2
A = π(49)
A = 49 π sq cm.
A = 49(3.14)
A = 153.86 sq cm.
r
Remember:
π = 3.14
Find the area of the circle
15 in
A = πr2
A = π(15)2
A = 225 π sq in
A = 706.5 sq in
A = πr2
A = π(0.3)2
A = 0.09 π sq m
A = .2826 sq m
0.3 m
9 cm
A = πr2
A = π(9)2
A = 81 π sq cm
A = 254.34 sq cm
Try it on your own!
P. 454 #4 - 6
4. 201 sq in
5. 28 sq in
6. 113 sq ft
Find the radius
A = 49 π cm2
A = πr2
49 π = πr2
49 = r2
r = 7 cm
A = 314 ft2
A = πr2
314 = 3.14r2
100 = r2
r = 10 ft
Find the radius
C = 2 πr
15 π = 2 πr
C = 52 m
15 = 2r
r = 7.5 ft
C = 2 πr
52 = 2 πr
26 = πr
r = 8.28 m
C = 15 π ft
Practice at your desk!
P 457 # 27 - 29
27. 4 units
28. 24 units
29. 6 units
Area of a Sector
Central Angle: angle whose vertex is the
center of a circle
Sector: region or part of a circle formed
by a central angle
Area of a Sector
Area = πr2 x central angle / 360
A = π(5)2 x 120 / 360
A = 3.14(25)x120/360
A = 26.167 m2
5m
1200
Find the area
A=
πr2x165/360
A = 3.14(11)2x165/360
11 in
A = 3.14(121)(165)/360
A = 174.139 sq in
1650
Try it on your own!
P 455 # 9 - 11
9. 2 sq ft
10. 20 cm2
11. 42 sq in
Assignment
P 456 # 10 - 20, 24 - 26, 33-35