# Cyclic Quadrilaterals

- Author:
- Tim Brzezinski, GeoGebra Team German

**Definition:**A

**cyclic quadrilateral**, by definition, is any quadrilateral that can be inscribed inside a circle. That is, all 4 vertices of a cyclic quadrilateral always lie on the circle itself. Mess around with the applet for a couple of minutes, and then answer the questions that follow.

Based upon your observations,

*what can you conclude about both pairs of opposite angles of any cyclic quadrilateral?*Prove your assertion true using a theorem previously learned. Explain fully why what you've observed in the applet above is true.