Each student makes a triangle on a geoboard, ensuring a variety of types of triangles. Students then record the triangle twice on the geoboard paper. The students measure the angles of their triangles with a protractor and mark the angle measurements on both copies. Students cut off the angles of one of the triangles. They rotate the angles around a point. Students will see that the three angles of any triangle, when rotated around a point, will always result in a straight line. Since they know that straight lines are 180 degrees, they see that the angles of all triangles will add up to 180 degrees.

Here is an example from Mr. Greenman's Class:

Download the assorted triangles below instead of having students make their own on the geoboards.

 

Indicators:

2.6.1.1 use a variety of triangles and quadrilaterals to draw conclusions about the sum of the measure of their interior angles

3.5.2.1 use protractors to measure angles.