1. Section 3-3 Parallel Lines and the Triangle Angle-Sum Theorem

2. Activity #1 1 2 3 1 2 3 1 2 3

3. Activity #2 1 1 2 2 3 3

4. Formal Proof 1 A B C 45 23 DE Triangle Angle-Sum Theorem: The sum of the measures of the angles of a triangle is 180 ˚.

5. Formal Proof 2 A B C 13 Triangle Exterior Angle Theorem: The measure of each exterior angle of a triangle equals the sum of the measures of its two remote interior angles. D4

6. Classifying Triangles Classify by Angles 60 ˚ Equiangular Acute Right Obtuse Classify by Sides EquilateralIsoscelesScalene

7. Example 1 67 ˚ 48 ˚ x

8. Example 2 x z y 70 ˚

9. Example 3 :classify by angles and sides 2 4 120 ˚ 5

10. Example 4 : 125 ˚ X

11. Example 5 : A triangle with a 90˚ angle has sides that are 3 cm, 4 cm, and 5 cm long. Classify the triangle by its angles and sides.

12. Example 6: 70 ˚ 42 ˚ y

13. Example 7: 90 ˚ 76 ˚ x

14. Example 8: 2x + 28 32 ˚ 4x

15. Example 9: 5x + 40 10x 3x − 4

16. Example 10: x 160 ˚ 125 ˚