# Activities

• • • ##### Subject Area

• Math: Geometry: Circles
• Math: PreAlgebra: Geometry

• ##### Author 9-12

60 Minutes

• ##### Device
• TI-83 Plus Family
• TI-84 Plus
• TI-84 Plus Silver Edition
• ##### Software

Cabri Geometry™
TI Connect™

• ##### Accessories

TI Connectivity Cable

• ##### Other Materials
This is Activity 16 from the EXPLORATIONS Book:
Exploring The Basics Of Geometry With Cabri

## Radius, Diameter, and Circumference of a Circle

#### Activity Overview

In this activity, students will learn the basic concepts of the circle. They explore the relationship between the diameter and the radius, and between the diameter and circumference of a circle. They also get familiar with the Greek symbol π (pi).

#### Before the Activity

Install the Cabri™: Jr. App on the students' graphing calculators using one of these two methods:

• TI-Connect™,  a TI Connectivity Cable, and the Unit-to-Unit Link Cable
• TI-Navigator™  "send to class" feature
• See the attached PDF file for detailed instructions for this activity
• Print pages 65 - 67 from the attached PDF file for your class
• #### During the Activity

Distribute the pages to the class.

• Construct a circle
• Draw the radius by constructing a segment from a point on the circle to the center of the circle
• Construct the diameter by drawing a segment from a point on the circle to another point on the circle, and passing through the center
• Measure the length of the radius and the diameter
• Observe that the length of the radius is half the length of the diameter
• Alter the circle and verify the relationship
• Calculate the circumference of the circle
• Calculate the ratio of the circumference to the diameter, and observe that the ratio is equal to π (pi) or 3.14
• Calculate the circumference and radius for other circles and examine the ratios
• Note that of any circle, circumference is equal to π (pi) times diameter and also equal to 2 radii times π (pi)
• #### After the Activity

Review student results:

• As a class, discuss questions that appeared to be more challenging
• Re-teach concepts as necessary