Use this applet for class demonstration or for student self directed learning.
Other useful applets for checking on irregular polygons
Interior Angles :
http://tube.geogebra.org/material/show/id/7100 (Angles already displayed, to sum up)
http://tube.geogebra.org/material/show/id/26422 (Construct own triangles by joining vertices, discover formula for sum)
Exterior Angles :
http://tube.geogebra.org/material/show/id/26430 (Angles already displayed, to sum up)
http://tube.geogebra.org/material/show/id/36302 (Move slider to see visual impact of sum of exterior angles in polygon)
