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Current time:0:00Total duration:5:25

Video transcript

so we've already spent some time introducing ourselves to the idea of adding or subtracting positive and negative numbers and what I now want to do in this video is do a bunch of examples using the adding negative numbers on the number line exercise on Khan Academy so that we can think about different ways to a model or think about or visualize adding and subtracting positive and negative numbers so this question asks us which number line model represents the expression negative 2 plus negative 7 and a number line model seems like this really fancy thing but they're just saying well which of these which of these diagrams which of these drawings are a way to think about what negative 2 plus negative 7 is so let's think about it someone has I'm want to start at negative 2 and then to that I want to add negative 7 so let's see what's going on here so this is this number line model I guess we could call it it's saying it's starting this is positive 2 right over here this to the right of 0 and then it and then it subtracts 7 so this is the model that would be 4 positive 2 plus negative 7 or 2 or positive 2 minus 7 but that's not what we have over here we have negative 2 minus 7 this one right over here let's see this first I guess you could say this thing this this this line this arrow it's going 1 2 3 4 5 to the left of 0 so starting at negative 5 and then it goes another 1 2 3 4 to the left of that so you could view this as negative 5 minus 4 or you could view this as negative 5 plus negative 4 but that's not what we have up here and then this last one hopefully this is the answer otherwise there'd be some mistake in the exercise let's see we have this first arrow that takes us two steps to the left of 0 so 1 2 so this is negative 2 and then we take another 7 steps to the left of that so 1 2 3 4 5 6 7 so you can think of this arrow is representing negative 2 and then we're going to do that so we're starting from this tip of this our we're going to add a negative seven which makes us move seven to the left again to get to negative nine so that's definitely the model that represents that expression let's keep going all right which number line model represents the expression six plus negative two so here they've taken our number lines but they've made them they made them go up and down they've made them vertical so we want to one way to think buddy I want to start at six and then I want to add negative two so on a vertical number line like this and it seems like we're increasing as we go up I want to start at six above zero so I want to start at six above zero but then I want to add negative two which should take me two steps back down this one takes me two steps even higher so this is this one right over here is positive six plus 2 or positive six plus positive two this one over here we're starting at positive 4 and then we're adding two so that's not what we're talking about this one over here this big error this tall arrow this is going it's pointing up it gets us to positive six the tip of the arrows right at positive six but then we go back down to so this would be positive six plus negative two you could think of this hours representing positive six and then this arrow is representing negative two it takes us to down so if you take positive six plus negative two you the positive six so then the tip of your arrow from there you could start the negative two and you go to back down and you end up at four so it's definitely that one right over there this is surprisingly fun let's keep going which number line model represents the expression negative 2/5 plus 4/5 so we're going to start at negative 2/5 which is going to be its negative it's going to be to the left of zero so let's see this this arrow right over here this is negative 4/5 so that's not going to be quite right so the second choice what's going on here so if I start at zero and then I'm going neat I'm going to fist to the left of zero so that's negative 2/5 and from that I'm going to the right 1/5 2/5 3/5 4/5 so this looks right this looks like negative 2/5 so I'm starting at 0 I'm going to the left 2/5 so it's negative two fists and then I go to the right for fists from that tip so I'm from negative two-fifths I'm adding four fists so that looks right let's do one more this is this is exciting and even if you don't find it fully exciting anything you do you should convince yourself it's exciting you'll do a better job which number line model represents the expression five and a half plus negative three so five this is positive five and a half so I want to say I want to go five and a half to the right of zero this is this is five and a half to the left of zero so that's negative five and a half that's not what I want to deal with so this one looks interesting this one now this is actually five and a half to the left of zero as well so it's probably going to be this but let's just look at it so let's see we go five and a half to the right of zero so this is five and a half and then from that I go three left so that inde and it gets me to two and a half so it's this choice right over there exciting