# Overview

## Parts of a Circle

Developed by Paul Hartzer, Spring 2012
Purpose: This is the second lesson in a unit on the parts and relationships within a circle. The basic textbook is Glencoe’s Geometry, Michigan Edition.

# Objectives and Materials

## Objectives

Learn the terms for line segments within a circle: Radius, diameter, chord.
Understand how the radii/diameters of overlapping circles overlap.Identify the pattern of the perimeters of polygons of increasing numbers of sides.

## HCSEs

G1.6.2 Solve problems and justify arguments about chords and lines tangent to circles.

## Resources and Materials Needed

A ruler with a piece of sticky tack, a pencil, and a fine point white board marker (makeshift compass).
Basic or graphing calculators, one for each student

# Lesson

## Introduction and Lesson

Begin by having students consider the skeeball question (“Nested Circles”). This is meant to demonstrate the use of circles in a real world environment most of the students are likely to be familiar with (skee-ball is popular at Chuck E. Cheese’s, for instance).
Once the introductory exercise has been discussed, introduce the basic terminology for line segments (“chord”, “diameter”, “radius”). Make sure that students understand that a diameter is a special kind of chord. To reinforce the concept of equidistance of points in a circle, use the makeshift compass, as follows: Take a standard ruler with multiple holes. Place a piece of sticky tack behind one of the holes for friction and hold the ruler in place with a pencil through this hole. Place the marker through another hole and draw a circle.
Work with students to complete the exercises in the textbook involving radius calculation, overlapping circles, and the relationship between circumference, diameter and radius.

## Practice and Closure

Have students complete the “Check for Understanding” practice exercises.
Review the material and assign some problems for homework.