Videos and lessons to help Grade 8 students learn how to use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles.

*For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so*.

Common Core: 8.G.5

- I can define similar triangles.
- I can define and identify transversals.
- I can identify angles created when a parallel line is cut by transversal (alternate

interior, alternate exterior, corresponding, vertical, adjacent, etc.). - I can justify that the sum of the interior angles equals 180. (For example, arrange

three copies of the same triangle so that the three angles appear to form a line). - I can justify that the exterior angles of a triangle is equal to the sum of the two

remote interior angles. - I can use Angle-Angle Criterion to prove similarity among triangles. (Give an

argument in terms of transversals why this is so).

Related Topics:

Common Core for Grade 8

Common Core for Mathematics

More Math Lessons for Grade 8

Corresponding Angles

In this lesson, you will learn how to find the measurements of angles created when parallel lines are cut by a transversal by using corresponding angles.

Similar Triangles

In this lesson you will learn how to find the side length of a triangle by using the angle-angle criterion of similar triangles.

Vertical and Adjacent Angles

In this lesson, you will learn how to find the measure of angles when parallel lines are cut by a transversal by using vertical and adjacent angles.

Alternate Interior angles and Alternate Exterior angles.

In this lesson, you will learn how to find the measurements of angles created when parallel lines are cut by a transversal by using alternate interior angles and alternate exterior angles.

Angle in a Triangle

In this lesson you will learn how to find the measure of an angle in a triangle by using the other two angles.

8.G.5 - Angle Relationships.

Transversals and Right Angles 8.G.5

Angle Relationships with Parallel Lines - Part 1

This video discusses the three angle relationships, with regards to parallel lines, that are congruent to each other. These angle relationships are alternate interior angles, alternate exterior angles, and corresponding angles. I also go into detail explaining the basics behind transversals and parallel lines.

Angle Relationships with Parallel Lines - Part 2

This video discusses the two angle relationships, with regards to parallel lines, that are supplementary angles to each other. These angle relationships are consecutive interior angles, otherwise known as same-side interior angles, and consecutive exterior angles, otherwise known as same-side exterior angles.

Angle Relationships with Parallel Lines - Part 3

This video explains how to solve the harder types of problems when it comes to angle relationships and parallel lines.