1. Warm Up
Triangle Activity

2. 4.2 Angles of Triangles
Unit 2 Chapter 4

3. Triangle Angle Sum Theorem
The Triangle Angle-Sum Theorem can be used to determine the measure of the third angle of a triangle when the other two angle measures are known.

4. Auxiliary Line
an extra line or segment drawn in a figure to help analyze geometry relationships.

5. Example 1
SOFTBALL The diagram shows the path of the softball in a drill
developed by four players. Find the measure of each numbered angle.

6. Exterior & Interior Angles
Exterior Angle: an angle formed by one side of the triangle and the extension of an adjacent side. Each exterior angle of a triangle has two remote interior angles that are not adjacent to the exterior angles
Ð4 is an exterior angle of ∆ABC.
Its two remote interior angles are Ð1 and Ð3.

7. Exterior Angle Theorem

8. Flow Proofs
A flow proof uses statements written in boxes and arrows to show the logical progression of an argument. The reason justifying each statement is written below the box. See below for an example.

9. Flow Proof for Exterior Angle Theorem

10. Example 2
GARDENING Find the measure of ÐFLW in the fenced flower garden shown.

11. Corollary
A theorem with a proof that follows as a direct result of another theorem.

12. Triangle Angle-Sum Corollaries

13. Example 3
Find the measure of each numbered angle.

14. Exit Slip