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

A CO

B If the diameter is 18 inches, find the radius.

R = ½ 18 in.

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 perimeterof a polygon.

c dThe number is about 3.14 or 22/7.

1.4 m

Find the circumference of the circle. Use 3.14 for pi.

C = 3.14 (1.4 m)

C = 4.396 m

c d

Now try a fraction for the Diameter.

3/4 m

Find the circumference of the circle. Use 22/7 for pi.

c d22 6

7 4c

3 3 6

4 4 4

D = 2r 2

3

7

22

11 3

7 1= =

33

7

547

c m

Find the perimeter of this figure. Use 3.14 for pi.

7.1 yd

3.2 yd

7.1 7.1P d

2 half circles makea whole circle.

P = 7.1 + 7.1 + 3.14(6.4)

3.2+ 3.2 6.4

We need the Diameter.

P = 7.1 + 7.1 + 20.096

P = 34.296 yd

20.296 7.1+ 7.1 34.296

Area of a circle page 535

The area of a circle with radius of length r is given by

2A r14cm

Find the area of the circle using22/7 for pi.

2214 14

7A

22196

7A

28

A = 616 cm2

28x 22 56 560 616

3.14x 1961884

282603140061544.

Decimal point

Using 3.14

Area of a circle page 535

The area of a circle with radius of length r is given by

2A r 5km

Find the area of the circle using22/7 for pi.

225 5

7A

22 25

7 1A

25x 22 50 500 550

550

7A 24

787km

3.14x 25157062807850.

Decimal point

Using 3.14

The area of a circle with radius of length r is given by:

2A r 10.4 cmFind the area of the circle using

3.14 for pi.

10.4 x 10.4

4160000

10816.

A = (3.14)(10.4)(10.4)

10400

10816.x 3.1443264

10816032448003396224. 2cm

Which is larger and by how much: a l0 ft square flower bed or a 12 ft diameter flower bed?

10 ft12 ft

A = S x S

A = 10 x 10

A = 100 sq ft

2A r

r = 6

3.14 6 6A 3.14 36A

A = 113.04 sq ft 113.04 - 100.00

13.042ft

The 12 ft diameterflower bed is larger by