We've now done a proof and a demonstration that the sum of the interior angles of any triangle will equal . This is a big idea- it will come up again and again throughout the year. First we're going to look at a dynamic measure of this fact, and then we'll look at an alternative proof of this theorem.

Directions

1) In the applet below, use the polygon tool to construct an arbitrary polygon.
2) Use the angle tool to measure each angle.
3) Notice that the angles have appeared in the column at left. In the input field, find the sum of the angles. You will probably enter .
4) Move your points around. Notice that each individual angle measure changes, but the sum of the angles remains .