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Current time:0:00Total duration:8:32

Video transcript

welcome to the video on basic subtraction let's do a little bit of review of basic addition first if I said 4 plus 3 what did this mean 4 plus 3 what did that equal well there were a couple of ways we could have viewed this we could have said I had 4 of something let's say I had 4 circles or I don't know I had 4 lemons for for breakfast so 1 2 3 4 lemons for breakfast and let's say I had another 3 lemons for lunch 1 2 3 and so you could view 4 plus 3 as how many total lemons did I have I'm adding 3 to 4 so how many total did I have what's 1 2 3 4 5 6 7 so I had a total of 7 lemons another way we could have viewed that is we could have drawn our number line and I'll draw it in yellow because we're that's not wide enough I'll draw it in yellow because we're talking about lemons so let's say that's our number line and if I start at the number let me draw all of the numbers 0 1 2 3 4 5 6 7 so you could you could think about this as saying well we're on the number line we start at the number 4 alright that's this number 4 and we're adding 3 to it so we'll increase along the number line by 3 so we'll go 1 2 3 and you end up at 7 so you could say if I have 4 and I get 3 more I get 7 or if I increase 4 by 3 I also get 7 so what's subtraction now now because that's what this video is about I shouldn't waste all of our time talking about addition so let's take the example of 4 minus 3 what is that equal to let me switch colors just to keep interesting so what is four minus three equal to subtraction or minus is the opposite of addition so in addition you're doing something more you're adding I don't want to use the word adding to define addition but that's what you're doing I had four lemons and then I had three more in subtraction you're taking away so this example if I started with four lemons let's add four lemons on a plate if I'm subtracting three if I'm saying minus three instead of adding these three here and getting seven I'm going to take three away so maybe I'm eating them or maybe I'm giving them to you for in exchange for watching these videos so to take away three from this for let's say this one goes away this one goes away and this one goes away how many lemons would we have left well this is the only one that I haven't crossed out so we would have 1 lemon left one lemon left and this would be the lemon that's left didn't have to be that one I could have crossed out any of the three another way to view that let's draw the same lemon colored number line say that this is the number line right here and I'll draw all the same numbers so that's 0 1 2 3 4 5 6 7 of course the number line keeps going there's no largest number you can imagine any any number that you can think of I can think of one higher than that so there is no largest number so that's why we draw the arrow there I could never draw the entire number line but anyway back to subtraction so we're starting at 4 lemons right when we added 3 plus 3 we went to the right 4 spaces on the number line and that's because the right is increasing value so we went from 4 to 5 that was one more 5 to 6 was 2 more and 7 was 3 more now we're taking away from 4 so what do we do what would you think we do well since we're taking away we're going to decrease the total number of lemons we have right so we take away one we get two three take away to get two to take away three we took away three right so we'll go back one two three along the number line and we'll end up at one and that's this one right here so just to review addition is you're doing more of something subtraction is you are taking away if you think about it on the number line addition is increasing along the number line by bad amount so in this case we increased along the number line by three and so we went from four to seven in the subtraction case we decrease back on the number line so we decrease by the amount that you're subtracting so in this case we decreased by three so we went back one two three and we had one and the other way to view it if I four or something if I give three away or if I ate three of them or if I I don't know what I did to three of them if I lost three of them I would have one left now let me show you some interesting things about subtraction so we know that 4 minus 3 is equal to 1 let me show you something else that's interesting what is 4 minus 1 well we could we could do we could use either example let's do the lemon example if I had let's do apples now I'm bored of the lemons let's add 1 1 2 3 4 I have a new pen sometimes it doesn't draw right let's add 4 apples right this is the example we're dealing with and I were to eat one of them so one of them were to go away how many apples would I have left well 3 1 2 3 so 4 minus 1 is equal to 3 and if we did it on the number line if we started at 4 and we subtracted 1 we took 1 away so we're going to become 1 smaller we go back 1 we get 3 either way works but isn't this interesting 4 minus 3 is equal to 1 and 4 minus 1 is equal to 3 that you might say well is is this you know did I pick the numbers just so it worked well it turns out that it's always true for if some well I don't want to get too technical but this is we're already touching on unsub something that you'll learn later on maybe an algebra one day but actually don't want to go into that right now so where does this come from well this is also based on the fact that three plus one I didn't want to confuse you so I apologize if I did but I'll show you another interesting thing what is 3 plus 1 3 plus 1 is equal to 1 well that's easy you know that from basic addition you can start on the number line at 3 and add 1 do it and where do you end up you end up at 4 3 plus 1 is equal to 4 or you could have started at 1 on the number line and added 3 1 2 3 and you would have also ended it up at 4 right so we also know that you could have switched this either way both of those are equal to 4 what do you see here well there's there's a bunch of things I've written here and they all kind of relate to each other one plus three is equal to 4 3 plus 1 is equal to 4 4 minus 1 is 3 essentially 4 minus 1 and getting 3 is the exact same you're saying the same thing as 3 plus 1 is equal to 4 right this statement says if I add 1 2 3 I get 4 this is saying if I take away one from 4 I get 3 so if I start at 4 and move back I get 3 this is saying if I start at 3 and I move up 1 I get 4 hopefully that gives you a little bit of intuition about what subtraction is in the next video I'll just do as many basic subtraction problems I can do in 10 minutes and and then you'll be ready to do the exercises see you soon