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LET’S START BY LOOKING AT SOME

FEATURES OF CIRCLES!

CIRCLES

CircleA circle is a round figure with all of the points ON the circle the same distance to the center.

A

B

C

D

E

CENTER: The point in the middle of the circle. Click for effects

Diameter: The distance across the circle, through the center. (click for effects)

Radius: The distance from any point on the circle to the center. (click for effects)

Diameter is represented by “d”

Radius is represented by “r”

RADIUS AND DIAMETER

SOME CONCLUSIONS

Let’s look at the

relationship between

radius and diameter!

How are the Radius and Diameter Related?

Diameter

Radius

SO….The radius is half the length of the diameter.

The radius only goes HALFWAY through the circle.

Which means the diameter is twice as long as the radius.

d =2r

r = 1/2d

The diameter goes all the way through the circle.

Let’s Practice!

IDENTIFY THE RADIUS AND DIAMETER OF

CIRCLES

Identify the Length of the Radius

r = 7 cm

7 cm

Distance from a

point on the circle

to the center!

Identify the Length of the Diameter

d = 2r = 14 cm

7 cmThe

diameter is twice as

long as the radius!

Identify the length of the radius

r = 9 cm

9 cm

Distance from a

point on the circle

to the center!

Identify the length of the diameter

d = 2r = 18 cm

9 cm

The diameter is

twice as long as the

radius!

Identify the length of the radius

r = 6 cm

The radius is half the length of

the diameter!

Identify the Length of the Diameter

d = 2r = 2*6 = 12 cm

How will you find

the length of the

diameter?

Identify the Length of the Radius

r = 10 cm

How will you find

the length of the

radius?

NOW LET’S FIND OUT WHAT CIRCUMFERENCE IS!

NICE JOB SO FAR!

Circumference is the distance around a circle. (Click for effects.)

To calculate the circumference:

diameter

rdC 2r = radiusd = diameter

Multiply the

diameter by pi, which is about 3.14!

What is Circumference?radius

Let’s Practice!

FIND THE CIRCUMFERENCE OF

CIRCLES!

Find the circumference

C = cm68.3714.31212

Multiply the diameter by pi, which is about 3.14!

d r2C = =

Find the circumference in terms of pi

C = =

12 cm

r2 12

Don’t use 3.14 for pi,

use the symbol!

d r2C = =

Find the Circumference

cm96.4314.3141472

Multiply the diameter by pi, which is about 3.14!

d r2C = =

Find the circumference in terms of pi

r2 1472 dC = = = cm

Don’t use 3.14 for pi,

use the symbol!

d r2C = =

Find the Circumference

cm52.5614.3181892

d r2C = =

Multiply the diameter by pi, which is about 3.14!

Find the Circumference in Terms of Pi

C = = = cmr2 1892 d

Don’t use 3.14 for pi,

use the symbol!

d r2C = =

NOW LET’S FIND OUT WHAT AREA IS!

You’re Doing Great!

Area of a CircleThe area of the circle is the amount of space that changes color. (click to see).To Find the Area:

2rA

r

d

r = radiusd = diameter

Square the radius and multiply by

pi!

FIND THE AREA OF CIRCLES!

Let’s Practice!

Find the Area

A = 22 86.15314.349497 cm

2rA =

Square the radius and multiply by

pi!

Find the Area in terms of pi

A = = 49722r 2cm

Don’t use 3.14 for pi,

use the symbol!

2rA =

Find the Area

A = 22 96.20014.364648 cm

2rA =

Find the radius first! Square it,

then multiply by pi!

Find the Area in terms of pi

A = 6482 2cm

Don’t use 3.14 for pi,

use the symbol!

2rA =

Find the Area

A = 22 31414.310010010 cm

Find the radius first! Square it,

then multiply by pi!

2rA =

Find the Area in Terms of Pi

A = 100102 2cm

Don’t use 3.14 for pi,

use the symbol!

2rA =

Find the Area

A = = 22 5.7814.325255 cm 2r

Find the radius first! Square it,

then multiply by pi!

2rA =

CIRCUMFERENCE AND AREA

Let’s Review!

CIRCUMFERENCE of a Circle

1. What is the circumference of a circle?A)Distance Around the CircleB)Amount of Space on the Circle

2. To find the circumference, you need to know or calculate the __________.

A)RadiusB)Diameter

B) Diameter

A) Distance Around

AREA of a circle

1. What is the area of a circle?A)Distance Around the CircleB)Amount of Space on the Circle

2. To find the area, first you need to know or calculate the…?

A) RadiusB) Diameter

A) Radius

B) Amount of Space

THE END!

GREAT JOB!Now you know all about

circles!