Khan Academy
Khan Academy: Angles in a Triangle Sum to 180 Degree Proof
Learn the formal proof that shows the measures of interior angles of a triangle sum to 180 degrees.
Khan Academy
Khan Academy: Sum of Interior Angles of a Polygon
Learn how to find the sum of the interior angles of any polygon.
Sophia Learning
Sophia: Finding a Missing Angle: Lesson 2
This lesson will go over the steps involved in finding the missing angle of a given triangle.