Introduction & Installation

Circles A 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

Circles The distance across a circle through the center is called the diameter. A diameter.

Circles The radius of a circle is the distance from the center of a circle to any point on the circle. A radius

Circles If 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 2 radii = 1 diameter

Circles The distance around a circle is called the circumference. A

Circles If 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

Circles Dividing 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

Pi

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

Pi & GeoGebra OPEN the GeoGebra PI file.

Pi & GeoGebra

Formulas Area and Circumference To find the area of a circle: πr 2 To find the circumference of a circle: 2πr or πd

Circumference Find the circumference:

Area Find the area:

Area Find the area:

Circumference Find the circumference: