 This video is called Parallel Lines with the Transversal. I talked a little bit about Parallel Lines in the last, I'm sorry, I talked a little bit about transversals in the last video. Let's give a formal definition on it. A transversal is a line that intersects two others. A transversal is a line that intersects two other lines. So in this picture, the transversal would be the red line that's going vertically, more or less vertically, at a bit of a diagonal. The black horizontal lines are not the transversals, but we do know that they're parallel to each other. The reason we know that is both of them have these little arrows here. Whenever you see those markings that indicates that the lines that have the markings are in fact parallel. So let's go ahead and take a minute and remind ourselves that when you have two parallel lines cut by a transversal, they create eight angles. One, two, three, four, five, six, seven, eight. And the cool thing is when the lines are parallel, we can use relationships what we know about vertical angles and linear pairs to fill in all the angle measures. Let's start with the angles that are vertical. We know vertical angles are congruent. So vertical from 118 will also be 118. Vertical from 62 degrees will also be 62 degrees. So now we only have four angles left to figure out. Two from my top intersection and two from the bottom. And the cool thing is is these two at the top, they're vertical. So once we know one, we'll know the other. And then down here at the bottom intersection, these two vertical are congruent. So once we know one, we'll have the other. So we have to use, to figure this out, we have to use what we know about linear pairs. Remember linear pair would be like these two angles right here or these two angles right here or maybe these two or maybe these two. Linear pairs are supplementary and they're adjacent. So let's think about it. A linear pair being supplementary, that means they add up to 180. So if you do 180 minus 118, you get 62 degrees. So anything that's a linear pair with 118 is also going to be 62. Then down here, I think I've seen a pattern. If these vertical are 62 and they're linear pairs, 62 and this is a linear pair, these are a linear pair, these two angles are a linear pair and so is this one, well, these missing guys must be 118 because 62 plus 118 equals 180. One other relationship to notice is all the four angles together make a circle and a circle adds up to 360 degrees. If you added all four of these angles up, you sure would get 360. Same down here, all four of these make a circle. They should add up to 360 and they do. So not only do the linear pairs add up to 180 and the vertical angles are congruent, but all four together add up to 360.