A Circle is a closed plane figure in which all the points are the same distance from the center.

FCenter

Circle F

A Chord is a line segment that connects two points on a circle.

Y

B

C

D

A

Chords: AB AC AD

The Diameter of a circle is a special chord that passes through the center of a circle.

E

B

C

D

A

Diameter: AC

The Radius of a circle is a line segment from the center of the circle to any other point on the circle.

Y

B

C

D

A

Radii: YA YC YD

(pluralform ofradius)

diamete

r

radius

The length of the radius is one half the length of the diameter.

If r = 2 cm, then d = 4 cm.

d = 2 x ror

r = d ÷ 2d (diameter) r (radius)

If d = 5 in, then r = 2.5 in.

You can use a

compass to construct a circle.Step 1: Open the compass to the length of the radius.Step 2: Put the tip of the compass at a point to be the center.Step 3: Rotate the pencil around the center.

P

compass

The Circumference is the distance around a circle.

C (circumference)

The Circumference of a circle is greater than its diameter. It isalways the same number of times asgreat. This number is called pi and

is represented by the symbol π. ( π 3.14)You can find the circumference by multiplying π times the diameter.( C = π x d )

C = π x d = 3.14 x 8 = 25.12 cm

8 cmE

F G

d = 8 cm

The circumference is 25.12 cm.

C = π x dC = 2 x π x r = 2 x 3.14 x 3 = 18.84 in

3 in

S T

r = 3 in

The circumference is 18.84 in.

Remember:

d = 2 x r

C = 62 mFind the diameter (d) to the nearest hundredth.

C = π x d62 = 3.14 x d62 ÷ 3.14 = dd = 19.75

The diameter is 19.75 m.

diameter

C = 52 inFind the radius (r) to the nearest hundredth.

C = π x d

52 = 2 x 3.14 x r52 = 6.28 x rr = 52 ÷ 6.28

The radius (r) is 8.28 in.

C = 2 x π x r

radi

us