GeoGebra: Polyon Interior Angles - Investigations
GeoGebra

Polygon Interior Angles: Discovery Learning Explorations for GeoGebra Classroom

GeoGebra: Quadrilaterals
GeoGebra

Properties of Special Quadrilaterals

Inside Mathematics: 9th & 10th Grade Math - Properties of Quadrilaterals
Inside Mathematics (The University of Texas at Austin, Charles A. Dana Center)

This lesson is about properties of quadrilaterals and learning to investigate, formulate, conjecture, justify, and ultimately prove mathematical theorems.