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.

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.

## Parts of Circles

#### Radius

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

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

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 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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