An interactive applet and associated web page that demonstrate the properties of the interior angles of a triangle.

The applet shows a triangle where the user can drag the vertices to reshape it. As it is being changed, the angles are continually updated, along with an expression showing that they always add to 180°

Applet can be enlarged to full screen size for use with a classroom projector. After use in the classroom, student can access it again from any web browser at home or in the library with no login required.

This resource is a component of the Math Open Reference Interactive Geometry textbook project at http://www.mathopenref.com.