Khan Academy: Geometry: Proof Sum of Measures of Angles in a Triangle Are 180

Show your class that finding angle measures is a regular calculation with a resource that provides 12 problems dealing with the measures of angles in regular polygons. Pupils use formulas for the sum of measures of angles in a polygon to...

Build confidence in proofs by proving a known property. Pupils explore two approaches to proving base angles of isosceles triangles are congruent: transformations and SAS. They then apply their understanding of the proof to more complex...

You know that triangles have angle sums of 180 degrees, but can you prove it? A brief video in the Socratica Geometry series highlights an informal geometric proof. It shows that the sum of the interior angles of a triangle is 180...

Video proving that the sum of the measures of the angles in a triangle are 180.

Are you working on finding the missing angles in triangles in your class? Consider this video. An instructor uses the triangle sum theorem to create an equation that helps her find the missing angles. She explains each step and writes...

Proof that the measures of vertical angles are equal based on properties of supplementary angles. [4:51]

The third angle's the charm. Viewers of a short video learn how to find the measure of the missing angle in a triangle. It uses an arithmetic process rather than an algebraic one, so younger learners have no trouble understanding the...

Demonstrates how to solve five different problems from a released California Geometry state test that involve finding the measures of angles in different polygons. This video also appears in the Geometry: Worked Examples section. [10:58]

Proof that if the legs of an isosceles triangle are congruent then the base angles are congruent and vice versa using triangle congruency postulates. The proof is written as a two-column proof. [9:03]

Explains how to use given algebraic expressions for the measures of angles in an isosceles triangle and apply knowledge of the triangle sum theorem and the knowledge that the base angles of an isosceles triangle are congruent to...