Introduction & Installation

CirclesA circle is a shape with all points the same distance from the center.

A circle is named by the center.

The circle to the left is called circle A since the center is A. A

CirclesThe distance across a circle through the center is called the diameter.

A

diameter.

CirclesThe radius of a circle is the distance from the center of a circle to any point on the circle.

A

radius

CirclesIf you place two radii end-to-end in a circle, you would have the same length as one diameter. This means the diameter of a circle is twice as long as it’s radius.

A

radius radius

2 radii = 1 diameter

CirclesThe distance around a circle is called the circumference.

A

CirclesIf you measure the distance around a circle (C or the circumference) and divide it by the distance across the circle through the center (d or the diameter), you will always come close to a particular value, depending upon the accuracy of your measurement.

A

CirclesDividing the circumference by the diameter is a ratio represented by . This ratio is called π.

Thus, for any circle, if you divide the circumference by the diameter, you get a value close to π .

A

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Cd

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Cd= π

Pi

PiThe value of π (pi) is approximately 3.14159265358979323846... We use the Greek letter (pronounced Pi) to represent this value. The number goes on forever.

A

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Cd= π

Pi & GeoGebra OPEN the GeoGebra PI file.

Pi & GeoGebra

Pi & GeoGebra

FormulasArea and Circumference

To find the area of a circle: πr2

To find the circumference of a circle: 2πr or πd

CircumferenceFind the circumference:

Area

Find the area:

Area

Find the area:

CircumferenceFind the circumference: