Transcript Slide 1

Circles 9.3
Page 532
Diameter
Is a segment that passes through the center of the circle and has
endpoints on the circle. Segment AC is the diameter. D = 2r
Radius
Is a segment with one endpoint on the center and the endpoint
on the circle. Segment OB is called the radius. R = ½ d
If the diameter is 18 inches, find the
radius.
R = ½ 18 in.
B
A
O
C
R = 9 in.
If the radius is 2 ½ ft, find the diameter.
D = 2r
D = 2(2.5 ft.)
D = 5 feet
The circumference of a circle is the distance around it.
Finding the circumference is similar to finding the perimeter
of a polygon.
c d
The number
 is about 3.14 or 22/7.
Find the circumference of the circle. Use 3.14 for pi.
1.4 m
c d
C = 3.14 (1.4 m)
C = 4.396 m
Now try a fraction for the Diameter.
Find the circumference of the circle. Use 22/7 for pi.
3/4 m
D = 2r
3 3 6
 
4 4 4
c d
22 6
c

7 4
22 3

7
2
5
c4 m
7
=
11 3

7 1
=
33
7
Find the perimeter of this figure. Use 3.14 for pi.
3.2 yd
2 half circles make
a whole circle.
7.1 yd
P  7.1  7.1   d
P = 7.1 + 7.1 + 3.14(6.4)
P = 7.1 + 7.1 + 20.096
P = 34.296 yd
20.296
7.1
+ 7.1
34.296
3.2
+ 3.2
6.4
We need the
Diameter.
Area of a circle
page 535
The area of a circle with radius of length r is given by
A r
2
Using 3.14
Find the area of the circle using
22/7 for pi.
22
A
 14  14
7
28
22
A
 196
7
A = 616 cm2
28
x 22
56
560
616
14cm
3.14
x 196
1884
28260
31400
61544
.
Decimal point
Area of a circle
page 535
The area of a circle with radius of length r is given by
A r
2
Find the area of the circle using
22/7 for pi.
25
22
A
55
x 22
7
50
22 25
500
A

550
7
1
550  78 4 km 2
A
7
7
5km
Using 3.14
3.14
x 25
1570
6280
.
7850
Decimal point
The area of a circle with radius of length r is given by:
A r
2
10.4 cm
Find the area of the circle using
3.14 for pi.
A = (3.14)(10.4)(10.4)
.
10816
x 3.14
43264
108160
3244800
. cm2
3396224
10.4
x 10.4
416
0000
10400
10816
.
Which is larger and by how much: a l0 ft square flower bed
or a 12 ft diameter flower bed?
10 ft
12 ft
r=6
A=SxS
A r
A = 10 x 10
A  3.14  6  6
A = 100 sq ft
A  3.14  36
113.04
- 100.00
The 12 ft diameter
2
flower bed is larger by 13.04 ft
2
A = 113.04 sq ft