1. 4-2 Identifying Triangles
Chapter 4
4-2 Identifying Triangles

2. Objectives
Classify triangles by their angle measures and side lengths.
Use triangle classification to find angle measures and side lengths.

3. What is triangle?
a triangle ( ∆) is a polygon with three sides. Triangles can be classified in two ways: by their angle measures or by their side lengths.

4. Classifying triangles by angles
Acute triangle : three acute angles
Equiangular triangle: Three congruent angles
Right triangle: One right angle

5. Classifying triangles by angles
Obtuse triangle: One obtuse triangle

6. Example Classify ∆ BDC by its angle measures B is an obtuse angle.
B is an obtuse angle. So BDC is an obtuse triangle.

7. Example Classify ∆ ABD by its angle measures.
ABD and CBD form a linear pair, so they are supplementary.
Therefore mABD + mCBD = 180°. By substitution, mABD + 100° = 180°. So mABD = 80°. ABD is an acute triangle by definition.

8. Student Guided Practice
Do problems 3-5 in your book page 227

9. Classifying triangles by sides
Equilateral triangle: Three congruent sides
Isosceles triangle: Two congruent sides
Scalene: No congruent sides

10. Example Classify ∆ EHF by its side lengths.
From the figure, So HF = 10, and EHF is isosceles.

11. Example Classify ∆ ACD by its side lengths.
From the figure, So AC = 15, and ACD is scalene.

12. Example
Find the side lengths of ∆ JKL.

13. Example
Find the side lengths of equilateral ∆ FGH.

14. Student Guided practice
Do problems 9 and 10

15. Application
A steel mill produces roof supports by welding pieces of steel beams into equilateral triangles. Each side of the triangle is 18 feet long. How many triangles can be formed from 420 feet of steel beam?

16. Closure
Today we learned about classifying triangles and solving problems involving triangles
Next class we are going to learn about finding the measure of the interior and exterior angles of a triangle.