How to Solve Triangle Problems Using the Exterior Angle Theorem

The Organic Chemistry Tutor

A comprehensive geometry tutorial that explains and demonstrates the Exterior Angle Theorem of triangles. The video begins by defining the theorem—that the measure of an exterior angle of a triangle is equal to the sum of the measures of its two remote interior angles—and visualizes these concepts using clear, hand-drawn diagrams on a digital whiteboard. The lesson progresses from simple numerical examples to more complex algebraic applications. Initial problems involve finding missing angle measures using basic arithmetic, reinforcing concepts like linear pairs and the Triangle Sum Theorem. As the video advances, the instructor introduces algebra, requiring students to set up and solve linear equations to find variables representing angle measures. The final segment challenges viewers with a problem involving quadratic equations, demonstrating how geometric principles often intersect with algebraic skills. This resource is highly valuable for middle and high school geometry classrooms. It provides scaffolded instruction that supports differentiation; struggling learners can focus on the initial numerical examples, while advanced students can engage with the algebraic applications. Teachers can use this video to introduce the theorem, model problem-solving strategies, or as a review tool before assessments involving triangle properties.