# GoGeometry Action 136!

- Author:
- Tim Brzezinski

Creation of this applet was inspired by a problem posted by Antonio Gutierrez (GoGeometry).
Even though you can move the colored vertices of the quadrilateral anywhere you'd like, this applet will work best if the quadrilateral remains

**CONVEX.**(If you don't remember what it means for a polygon to be convex, click here for a quick refresher).**Note:**The**blue**and**red**sliders control the measures of the angles with**blue**and**red**vertices, respectively. How would you, in your own words, describe the phenomena you observe here?**How can we formally prove that the 4 yellow points (soon to appear) ALWAYS LIE ON A CIRCLE?**