**Formula Determination**

Odom, Clark Collins H.S.

1-312-542-3500

**Objective**
1) Student will learn to determine the number of degrees in any convex polygon.
**Equipment and Material**
Overhead Projector
Geo-Boards
Rubber Bands
**Recommended Strategies**
Remind the students that the sum of the three interior angles of every triangle
has a measure of 180 degrees.
Establish a meaningful definition of a polygon. A polygon is a many sided
closed geometric figure.
Using the overhead projector have a transparent geo-board on the overhead
projector. Demonstrate the shapes of various polygons by stretching rubber bands
around the nails. Take another rubber band stretch it from one vertex to another non
adjacent vertex. This is to determine the number of triangles formed. Repeat this
operation with various sided polygons. The students will be recording their findings
on a chart. A sample chart is below. Prepare a sample chart to pass to each student
constituting of data each student should make an attempt to actively collect.
**Chart**
**No of Sides** **Triangles Formed** **No of Degrees in**

**each Triangle**
---------- ------------ ----------
---------- ------------ ----------
--------- ------------ ----------
. . .
After each student has collected the data, he should be able to derive the
formula (N - 2)180 which is the desired goal.
Analyze this formula for the student to illustrate how this is used to determined
the number of degrees in any polygon with (N) number of sides by the number of
triangles that was formed with the geo-boards and rubber bands.

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