GeoGebra 4 - Quadrilaterals

Directions: Click and drag the vertices to change the quadrilateral.
                      Then, click and drag the red slider to see the triangles.

Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and activated. (click here to install Java now)

Now go back and find out how to make this and why it works!
 

LFS, Created with GeoGebra