 This video deals with polygons. So the first thing that you want to make sure that you write down in your notes is what a polygon is. So this definition up here, a closed figure whose sides are all segments. And then we have a table here, which would be a good idea to copy and put into your notes. Because you do need to know these names of polygons, I know that a lot of these you do already know. But it would be good to have them in your notes. So if you're unfamiliar with any of these, you have somewhere that you can look to figure out what is a pentagon or how many sides does a nonagon have and so on. So you might want to pause this video and copy down this table to have in your notes. You'll notice that once you get past 12, N just is a, is really any number. So once you get past 12, if you have 13, for example, it's just called a 13-gon and you just put the number in front of G-O-N. Some more vocabulary that you have to be familiar with when you're dealing with polygons is these two words, concave and convex. So polygons can be one or the other and you see two examples here. Again, you might want to pause and the video and put this stuff into your notes. But what's really important to understand is when you have a convex polygon, no points of the lines are in the interior. So see how if I extend any of these segments, they don't go through the inside of the shape. Whereas over here, a concave polygon, some of the lines do pass through the inside. The original shape here, by the way, looks like this. If I copy it. So again, if I extend one of these segments, for example, and I'll just do this one, it goes right through the shape and that's what makes it concave. Another really important vocabulary word to know is what makes a polygon regular. So regular polygons, you'll notice three things here. Convex, so in order to be regular, a polygon has to also be convex and then these are the two really important pieces. All of the sides and all of the angles must be congruent, which remember means that they have to be the same. So probably one of the most common would be a square. All four sides are congruent and all four angles are all right angles, which means they're congruent. The other example videos that you're going to watch in this section have to do with perimeter circumference in area. So it's important that you understand what we mean when we say any of these three words. So again, you probably want to pause and copy down these definitions. Hopefully you're familiar with them, but if not, you want to make sure that you have them in your notes. So we have perimeter, which is the sum of the length of all sides. So if you're trying to find the perimeter of a square, for example, you have to add all four sides together. Circumference is the perimeter of a circle. So instead of using the word perimeter when we're talking about a circle, we use the word circumference and that is just talking about the distance around a circle. So if I were to go around a circle, I call that the circumference. And then area is considered the number of square units needed to cover the surface. So if I draw a little picture of a square, for example, perimeter would be adding the four sides. So one side plus this side plus this side plus this side. Area, however, if you fill this with square units, so whether it's square feet or square inches or whatever it is, when we talk about area, we're asking how many of these square units do we need to cover this whole square? So again, I'm not going to draw this all out, but that's what area is. What do we need? The number of square units do we need to cover the square or whatever shape you're talking about?