1. Lesson 3: Introduction to Triangles
Unit 3 Triangles
Lesson 3: Introduction to Triangles

2. Lesson 3: Introduction to Triangles
Triangle: A polygon with 3 sides

3. Vocabulary Sides: the segments that make the boundaries of the figure
Vertices: The points where the sides come together
Angles: formed by the joining of two sides of a polygon
Exterior Angles: the angles outside the triangle that are supplementary to the corresponding interior angle D

4. Classifying Triangles by their angles
Acute Triangle: all angles are _________
Obtuse Triangle: one angle is _________
Right Triangle: one angle is ___________
Equiangular Triangle: all angles are _____________
*also an _________ triangle!
Less than 90°
Greater than 90° but less than 180°
Equal to 90°
Equal to 60°
equilateral

5. Classifying Triangles by their sides
Scalene Triangle: no sides are _________
Isosceles Triangle: at least _________ sides are congruent.
Equilateral Triangle: all sides are___________
NOTE: An equilateral triangle is a special kind of isosceles triangle
congruent
two
congruent

6. Example Problems
Ex 1: Find x, QR, RS, and QS if QRS is an equilateral triangle. R 4x
2x + 1 Q S
6x – 1

7. Ex 2: Find x, JM, MN, and JN if JMN is an isosceles triangle.

8. Triangle Angle Sum
The sum of the angles in a triangle add up to ________________.
The sum of the acute angles in a right triangle add up to _______________.

9. Each angle in an equilateral triangle is the _________________ and measures _______________.
The measure of an exterior angle is equal to the sum of its remote interior angles.

10. Your Turn:

12. Homework
Unit 3 Lesson 3 Worksheet