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Sal discusses the difference between lines, line segments, and rays.  Created by Sal Khan.
Video transcript
What I want to do in this video is think about the difference between a line segment (a line segment) a line (a line), and a ray (and a ray). And this is kind of the pure geometrical versions of these, of these, things. And so a line segment is actually probably what most of us associate with a line in our everyday lives. So a line segment is something just like that (for a lack of a better word) a straight line, but why we call it a segment is that it actually has a starting and a stopping point. So most of the lines we experience in our everyday reality are actually line segments when we think of it from a pure geometrical point of view. And i know i drew a little bit of a curve here, but this is supposed to be completely straight. But this is a line segment. The segment, the segment is based on the fact that it has an ending point and a starting point, or a starting point and an ending point. A line if you're thinking about it in the pure geometric sense of a line, is, essentially, it does not stop. It doesn't have a starting point and an ending point. It keeps going on forever in both directions so a line would look like this (a line, would look like this) and to show that it keeps on going on forever in that direction right over there we draw this arrow, and to keep showing that it goes on forever in kind of the down left direction, we draw this arrow right over here. So obviously, y-you no I've never encountered something that just keeps on going straight forever (forever), but in math (thats the neat thing about math) we can think about these abstract notions and so the mathematical purest geometric sense of a line is the straight thing that goes on forever. Now a ray is something in between. A ray has a well defined starting point (so that's its starting point), but then it just keeps on going on forever. So then, so the ray might start over here, but then it just keeps on going (then it just keeps on going). So that right over there is a ray. Now that out of the way, let's actually try to do the Khan Academy module on recognizing the difference between line segments, lines and rays. And i think you'll find it pretty straight forward based on, on our little classification right over here. So let me get the module, let me get the module going. (let me, where did i put it? where did i put it? there you go!) Alright, so what is this thing right over here? Well it has two arrows on both ends, so it's implying that it goes on forever. So this is going to be a line. (lets check our answer.) Yeah! Its a line. (now it's taking some time, oh correct, next question) Alright now what about this thing? Wait once again arrows on both sides means that this thing is going to go on forever in both directions, so once again it is a line. Fair enough, lets do another one. Here, we have one arrow, so it goes on forever in this direction, but it has a well defined starting point. So it starts there and then goes on forever. And if you remember that's what a ray is. One staring point but goes on forever. So it goes, or more when you think about it, it goes on forever in only one direction. So that is a ray. So lets do another question. This right over here, you have a starting point and an ending point. Or u can call this the start point and the ending point but it doesn't go on forever in either direction. so this right over here, is a line segment. (there you go) So hopefully that, that gives you enough t-t-to work your way through this module. And you might notice that when i did this (when i did this) module right here, there is no video and that's exactly what this video is, it's the video for this module.