1. You will explain the definition of a circle, radius, chord, diameter, circumference and arc.

2. You will name and label the terms of a circle.

3. You will understand special relationship between the diameter and the circumference.

Lesson Objectives

TERMS OF A CIRCLETERMS OF A CIRCLE

Developed byIvan Seneviratne

A circle is a shape with all points in a plane at a fixed distance from the center. The given point is called the ‘CENTER’ of the circle. It is named by the center. The circle below is called circle A since the center is at point A.

A

What is a circle?

Centre

Radius or RadiiA ‘RADIUS’ of a circle is a segment

that has one endpoint at the center of the circle and the other endpoint on the circle. The radius is referred to in formulas as r.

ARadius

ChordA ‘CHORD’ of a circle is a segment that has

its end points on the edge of the circle. 

ARadius

Chord

DiameterA ‘DIAMETER’ of a circle is a chord

that contains the center of the circle. It is referred to in formulas as d.

ARadius

Chord

Diameter

CircumferenceThe CIRCUMFERENCE (perimeter) is

the distance around the circle and is referred to in formulas as C.

To calculate the circumference of a circle you multiply pi with the diameter or pi with twice the radius.  Circumference

ARadius

Chord

Diameter

ArcAn arc of a circle is a segment of the

circumference of the circle.

Radius

Chor

d

MajorArc

MinorArc

The Special RatioIf you measure the distance around

a circle and divide it by the distance across the circle through the center, you will always come close to a particular value, depending upon the accuracy of your measurement. This value is approximately 3.14159265358979323846...

We use the Greek letter (pronounced Pi) to represent this value. The number goes on forever. However, using computers, mathematicians have been able to calculate the value of to thousands of places.

Conclusion

… the distance around the Circle…

… its PERIMETER

Diameter

… the distance across the circle, passing through the centre of the circle

Radius

… the distance from the centre of the circle to any point on the circumference

This presentation is developed by Ivan Seneviratne © 2007, purely for personal use.

ivanthexplorer@yahoo.co.uk