Discovering Discovering Parallelograms Parallelograms

Lesson 1: Angles of Polygons
Objectives: SWBAT
1. Find the sum of the measures of the interior angles of a polygon
2. Find the sum of the exterior angles
Materials:
• Glencoe Mathematics text-Geometry Ch 8, pp 404-410
• Polygon angle worksheet discovery activity
Procedure:
I. Warm-Up activity: Find each angle in a triangle, if the angles have a ratio
of 4:5:6. Show your solution process.
II.
III.
IV.
After discussing the warm-up activity, connect to today’s lesson:
Q: How can you determine how many degrees are inside a polygon?
Hint: Try to draw as many triangles as possible by connecting a vertex to
each other vertex (see polygon activity).
Students will work in groups to draw polygons and connect vertices. They
should “discover” the Angle Sum Theorem: S = (n - 2)180
Compare interior to exterior angles (connect to prior knowledge of
“linear pair” angles). Then students should complete activity and compare
their findings.
V. Summary/Closure: How can your method be used without drawing the
polygon?
Examples:
1. Find the angle sum measure in a 20-gon.
2. Find an interior and exterior angle measure of a “regular” decagon.
VI. Assignment: textbook applications. Pg 407, #1-11.
6