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## Class 1

### Course: Class 1 > Unit 2

Lesson 2: What is subtraction?# Intro to subtraction

Sal talks about what it means to subtract. The examples he uses are 4-3 and 5-2. Created by Sal Khan.

## Want to join the conversation?

- What if the 2 numbers were switched? How can you even go below zero?(55 votes)
- Below zero there would be negative numbers(4 votes)

- How far does negative numbers go? Is it like normal numbers? Never ending?(20 votes)
- Yes, Negative numbers go on forever just on the other side of the number line.(18 votes)

- Why subtraction has this (-) sign?(11 votes)
- It is easier to use the (-) sign is stead of saying plus a negative.(6 votes)

- How would you represent negative numbers?(13 votes)
- Negative numbers represent the distance in the opposite direction on the number line. The negative symbol can be seen as a subtraction sign, because 0 - 5= -5.(9 votes)

- How do you write subtraction of negative numbers?? Like this: -2 - -1? It looks kinda weird to me.(6 votes)
- If you are talking about subtracting a negative from a negative it would look like that but with parenthesis. -2-(-1). To write one negative number ,for example, if you solve 6-7 that would be equal to -1.(13 votes)

- what if you did 4-5? how would that even be possible? would it be 1?(3 votes)
- That is a really good question. In reality you can't subtract more than what you have in the first place. For example, you can't start with 4 apples and then give away 5 of them.

However, there are cases in which we can "take away" more than what we started with.

Let's take the example above and think about it as steps. If you took 4 steps forward and then 5 steps backward, where would you be? You be 1 step behind where you started, which would be -1.(7 votes)

- I'm practicing adding and subtracting mentally, I have no problem adding numbers quickly in my head but when it comes to subtracting it takes a little longer... What are some methods or tricks I can learn in order to subtract quickly without hesitation?(2 votes)
- Here's a fast way to do 57 - 19.

19 is close to 20, so first do 57 - 20 to get 37, then add 1 back to get 38.

Have a blessed, wonderful day!(7 votes)

- who made subtraction? as like: who create the subtraction(2 votes)
- No particular person created subtraction. Subtraction originated from the concept of removing an amount of something from a collection.

Early man was able to grasp the concept of having a number of apples and then taking a certain number away and having less. Soon we understood that you can assign values or names to things

ie: this is "one" apple. Everything after that just comes down to what was used to represent a certain number of something, is it "five?" "a grunt" or "cinco." Then language or symbols became more universal and got specific enough that I can say,

"eleven minus seven" and you know that's four. Additionally, you can also see four of something in your head since you know the relation of numbers to tangible physical things.(4 votes)

- I don't understand anything! So what is 3-1?(2 votes)
- When you are subtracting one number from another, you are taking some of it away. One way to do this problem is by counting with your fingers.

First, hold three fingers up in the air. You want to subtract, or get rid of one of them, so you put one finger down. How many fingers are in the air now?

After counting, you see that you have two fingers left in the air. For this reason,`3 - 1 = 2`

(3 votes)

- Suppose a bigger negative number is subtracted from a negative lower number. Like - 2 - -8 =?(2 votes)
- this would change to -2 + 8 which equals 6(3 votes)

## Video transcript

Voiceover:Let's explore what it means to subtract numbers. So let's say that I want to figure out what 4, what 4 minus 3 is. 4 minus 3. So one way to think
about this, is you start with 4 objects. And so let me just draw 4 objects. So there I have 1, 2, 3, and 4. And when I ... So this is the 4 objects right over here. And when I say minus 3, or if I'm going to subtract 3 from the 4, one way to think about it is, I'm going to take 3 of these 4 objects away. So let's do that. So I'm going to take away 1, I'm going to take away 2, and I'm going to take away 3. Notice, I took away 1, 2, 3 objects. So how many do I have leftover? Well, if I start with 4 and I take away 3, I subtract 3, 4 minus 3, I am left with, I am left with, 1
object, right over here. So 4 minus 3 is equal to, is equal to 1. Fascinating. Let's do another one of these. Let's figure out what 5,
what 5, what 5 minus 2 is. And let's write it this way. Let's say we want to figure out something, some question mark, some question mark ... Actually, let me just clear this out. I'll just do it right over here, actually. So let's say we have some question mark. So there's some unknown
number right over here. So I'll just put a
question mark over here. And we say that unknown
number is equal to, is equal to 5, 5 minus 2. Minus 2. Minus 2. So what is this going to be? Well let's visualize it. This means I have 5 things and I'm going to take away 2 of them
and this is going to be what I have leftover after I start with 5 and take away 2. So I have 1, 2, 3, 4, 5 things. Now I am going to take away 2 of them. So I'm going to take away 1 and 2. So I took away 1, 2 objects. So how many do I have leftover? Well, I have leftover these, these ... Let me do this in a different color. I have leftover these purple
things right over here. So how many is that? How many do I have leftover? Well, I have leftover 1,
2, 3 of my original 5. So something is equal is 5 minus 2. That something is 3. 3 is equal to 5 minus 2. Let me just replace this with a 3. So we see that 3 ... Actually, let me do
the 3 in the same color as this green. So we see that 3, 3 is equal to 5 minus 2 or we could say that 5
minus 2 is equal to 3. Either way.