10.4 ­ Area of a Circle
Objective: Find the area of a circle.
Joseph Williams
GESD
jwilliams@gesd40.org
1

Radius ­ The
distance from the
center of a circle to
any point on the
circle.
Image: '1917 D penny'
www.flickr.com/photos/96586445@N00/188729710
2

pi ­ the ratio of the
circumference of a
circle to its diameter.
( 3.14 )
Image: 'Happy 3.14 Day3'
www.flickr.com/photos/17731548@N00/421636858
3

Area of a Circle
Area = (pi)(radius)2
2 2
A = r A = (3.14)r
3 cm
A = (3.14)32
A = 3.14 9
A = 28.26 cm2
4

12 ft.
19 ft.
6 ft
Find the area of the key and half circle above the free throw line.
5

Area of the key
A = bh
A = 19 12 = 228 ft2
Area of the half circle
Now we need to add
A = r2
them.
A = 3.14 62 = 3.14 36 = 113.04 ft2
We only need the area for half the circle! 228 + 56.52 =
113.04 / 2 = 56.52 ft2 284.52 ft 2
6