Some sequences of geometric objects change in predictable ways.

Some angles have special relationships based on their position or measure.

Polygons can be described uniquely by their sides and angles.

Draw a triangle on a piece of paper. Find the measure of each of the 3

angles in your triangle. Compare your angle measures with

those of your table group. What patterns or relationships do you observe?

Other than by measuring, how could we prove that the measures of the angles in a triangle add up to 180o?

Draw a triangle on a notecard. Cut out the triangle. Label each vertex of the triangle (close

to the vertex) Cut off the vertices of the triangle. Put them together. What do you get?

Use the Parallel Postulate (p. 604)› Look at figure 10.22 on p. 605

What do we know? What does this tell us?

4 2 5

1 3

l

m

A

B C

Line l is parallel to line m

Draw a triangle on a piece of paper. What is the sum of the angle measures of

the 3 exterior angles in your triangle?

Compare your results with those of your table group, and make a conjecture.

How could you prove your conjecture?

12

On a post-it, write a conjecture for what you think the rule might be for finding the measure of the exterior angles of any polygon.

How might you prove your conjecture?

Method 1: Cut out exterior angles

Method 2: Walk-and-Turn

Method 3: Use the relation with the sum of the interior angles

Quadrilaterals› In your groups, draw a quadrilateral.› Measure each angle.› Find the sum of the angle measures.› Record your result.

Repeat with a polygon with more than 4 sides.

Sides Total Angle Measure

Avg. Angle Measure

3 180° 60°

4

5

6

7

8

What pattern(s) do you notice?

What conjecture can you make?

How could you prove your conjecture?

There are 3 approaches for determining the measure of the interior angles of a polygon:› Find the triangles with a common vertex at

one vertex of the polygon› Find the triangles with a common vertex in

the center of the polygon› Walk and Turn

Section 10.3: #’s 15, 16, 22, 41, 42, 43, 47, 49

Turn in #’s 15, 16, 42, & 47