Illuminations
This discovery lesson has a java applet that allows students to rotate a polygon about the midpoints of sides to discover the sum of the interior angle measures.

Exterior Angle Sum Investigation
Students explore the exterior angle sum by manipulating a java applet and watching the angle measures change. Investigation questions are included on the site.

Sum of Interior & Exterior Angles of Polygons
This site has links for the sum of interior and exterior angles of polygons along with practice pages and teacher resources.

AlgebraLab Lesson
This lesson is very clear and provides good pictures and a few practice questions at the end.

Interior angle of a triangle
This applet demonstrates a surprising result about a triangle with vertices on two parallel lines.

Exterior angle sum
In this applet, students create random polygons and observe the sum of the angles.

Cool Math 4 Kids
This is a colorful development of the interior angle formula.

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