Parts of a Circle

A circle is a closed curve consisting
of all points in a plane which are at
the same distance from a fixed point inside it called the center.

A circle has many parts such as:
Center, radius, diameter, and chord.

This fixed point (equidistant)
inside a circle is called
center.

A circle is a closed curve
consisting of all points
in a plane which are at the
same distance (equidistant)
from a fixed point inside it.

A circle

A circle has one and only
one center.

O

Center

A line segment that joins any point on the circle to its center

is called a radius such as [OM].

M

A point on the circle

Center
O

 Radii (plural of radius) of a circle are equal in length.

[ON], [OM], [OL] and [OK]

 Infinite number of radii can be drawn in a circle.

Center

K

O

L

M

N

A line segment that joins any two points on the circle and

passes through its center is called a diameter such as [AB].

A

B

A circle

O

Center

A circle

O

M

 Infinite number of diameters can be drawn in a circle.

[AB], [PQ] and [MN]

 As the radii of a circle are equal in length, its diameters too

are equal in length.

B

Q

Center

P

A

N

M

The length of the diameter of a circle is twice the length of its radius.

So, Diameter = 2 x Radius

And the length of the radius of a circle is half the length of its diameter.

So, Radius = Diameter ÷ 2

Radius OM

Center

M

O

N

Radius ON

Diameter MN

Diameter MN = Radius OM + Radius ON

Radius OM = Radius ON

A line segment that joins any two points on the circle is

called a chord such as [AB].

O

B

A

A is a point on the
circle

B is another point
on the circle

Diameter is also a chord of the circle.

O

Chord CD

C

D

M

N

K

L

Diameter CD

The diameter is the longest chord.

O

Chord/Diameter CD

C

D

M
N

Chord MN

Chord KL
L

K

L

K

M

N

Chord MN

O Center

Infinite number of chords can be drawn in a circle

such as [MN], [GH] and [KL].

G

H

Multiple Choice

Choose the correct answer: The center of the circle is ___.

• O

• A

• B

• m

Multiple Select

Choose the correct answer: The radii of the circle are ___.

• [OB]

• [AB]

• [OC]

• [DE]

Multiple Choice

Choose the correct answer: The diameter of the circle is ___.

• [OB]

• [AB]

• [OC]

• [DE]

Multiple Select

Choose the correct answer: The chords of the circle are ___.

• [OB]

• [AB]

• [OC]

• [DE]

Multiple Choice

If the radius measures 3 cm, then the measure of the diameter is ___.

• 4 cm

• 1.5 cm

• 6 cm

Multiple Choice

If the radius measures 7 cm, then the measure of the diameter is ___.

• 3.5 cm

• 14 cm

• 7 cm

Multiple Choice

If the diameter measures 7 cm, then the measure of the radius is ___.

• 3.5 cm

• 14 cm

• 7 cm

Multiple Choice

If the diameter measures 12 cm, then the measure of the radius is ___.

• 12 cm

• 6 cm

• 24 cm