Common Core Math Vocabulary

Circle - Definition with Examples

What is a Circle?

Circle Definition: A circle is a closed two-dimensional round shape with no corners or edges. All points on the circle are equidistant from the center of the circle.

Circle

OA = OB = OC = OD = OE = OF = OG = OH

 

Circles Around Us

We see circles all around us. For example, objects like buttons, bicycle tires, dart boards, clocks etc are circular in shape. 

Circle Around Us

 

Parts of Circles

Radius

Radius of Circle

The line joining the center of the circle to the boundary is known as the radius of the circle. A circle has many radii. All radii of a circle are of equal length. In the circle above, PQ is the radius. 

Chord

Chord of Circle

A line segment joining any two points on the circle is known as the chord of the circle. In the circle above, CB is the chord.

Diameter

Diameter of Circle

A chord that passes through the center of the circle is known as the diameter of the circle. A diameter is always the longest chord in a circle. In the figure above, AB is the diameter as it passes through the center P.

Circumference

Circumference of Circle

Circumference is the length of the boundary of the circle. 

The circumference of a circle is about 3 times the diameter.

 

Relation between Radius, Diameter and Circumference

The diameter of a circle is twice its radius. 

In other words, the radius of a circle is half of its diameter.

Diameter = 2 x Radius

 

The circumference of a circle is approximately thrice the diameter of the circle.

Circumference = 3 x Diameter  (approximately)

 

This relationship helps to solve many geometry problems.

 

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