 In this introduction to section 6.1, we would just want to begin by reminding you of the difference between what is a polygon and what is not a polygon. Remember polygons are any two-dimensional shapes that have straight lines and the shape is closed and that's why these three figures over here would not be polygons. In this particular section, we're going to be talking specifically about angles of our polygons. Interior angles, as you could guess, are the angles inside the polygon and the exterior angles we'll be talking about are created if we extend the sides of the polygons out. The exterior angle will be that side that comes from that extended line to the edge of the polygon. You might want to note also that an interior and exterior angle create a linear pair and they would add up to 180 degrees. I have this box up here that we'll be filling out during the rest of these videos because specifically we are going to derive four formulas that talk about the sum of interior angles which would be all of these added up versus just one particular value of an interior angle and then the sum versus one exterior angle. During this whole chapter of videos, we will be referring to the quadrilateral family tree that you should receive from your teacher or you can download it from the Moodle site. We won't be filling this in today but have this handy as you watch the videos on going in this chapter and we're going to specifically fill in information that is pertinent to each of the quadrilaterals we will be studying.