Exterior Angle of a Cyclic Quadrilateral

This GeoGebra applet demonstrates a key geometric property of cyclic quadrilaterals: the exterior angle formed by extending any one side of the quadrilateral is equal in measure to its interior opposite angle. The dynamic construction features a circle with an inscribed quadrilateral whose vertices can be dragged along the circumference, allowing students to observe how the exterior angle and its opposite interior angle remain equal throughout all valid configurations. As users manipulate the shape, they can also see related properties such as the fact that opposite interior angles of a cyclic quadrilateral are supplementary. The applet supports interactive exploration, making it an ideal teaching aid for secondary school geometry lessons on circle theorems and quadrilateral properties. Students benefit by developing intuition for geometric relationships through visual confirmation rather than relying solely on abstract proofs, reinforcing both understanding and retention of this important theorem.