Triangle Exterior Angles
Investigating the exterior angle of a triangle.
Drag any of the vertices of the triangle around on the screen. Observe the angles of the triangle that have been measured for you. You should notice right away that the three interior angles always add to 180 degrees (of course!). Now, what do you notice about the marked exterior angle of the triangle? As you drag the triangle around, try to determine a relationship between this exterior angle and the interior angles.
Once you think you see the relationship, click the "show" box.
Write your own conjecture about the exterior angles of triangles.