How to Use the Triangle Exterior Angle Theorem

Miacademy & MiaPrep Learning Channel

This geometry video provides a clear and structured explanation of the Triangle Exterior Angle Theorem. Narrated by Justin, the lesson begins by defining key vocabulary such as "exterior angle" and "remote interior angle," ensuring students understand the components before learning the relationship between them. The video uses clear visual animations to demonstrate how extending a triangle's side creates an exterior angle and how to identify the corresponding remote interior angles. The core of the video focuses on the theorem itself: that the measure of an exterior angle of a triangle is equal to the sum of the measures of its two remote interior angles. The narrator connects this concept back to the Triangle Sum Theorem (angles adding to 180 degrees), reinforcing prior learning through a visual reminder of a "paper tearing" activity. This connection helps students understand *why* the theorem works, rather than just memorizing a formula. The lesson concludes with a guided practice problem that integrates algebra into geometry. Students are shown a triangle with angle measures represented by algebraic expressions (e.g., 4x + 7). The video walks through setting up the equation based on the theorem, solving for x, and—crucially—substituting the value back in to find the specific degree measure of the exterior angle. This makes the video an excellent resource for classrooms bridging geometry concepts with algebraic manipulation.