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## Early math review

### Unit 2: Lesson 1

What is addition? 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?(52 votes)
- Below zero there would be negative numbers(2 votes)

- How far does negative numbers go? Is it like normal numbers? Never ending?(17 votes)
- Yes, Negative numbers go on forever just on the other side of the number line.(14 votes)

- Why subtraction has this (-) sign?(10 votes)
- It is easier to use the (-) sign is stead of saying plus a negative.(5 votes)

- Can you take away 1/2 of something?(19 votes)
- Depends on the item. For example, you can cut a sandwich in half and take one piece.(2 votes)

- So then it's possible to do negative addition?(14 votes)
- It is possible, because you can add a number to a negative number to make it positive or closer to 0.(1 vote)

- who invented subtraction?(8 votes)
- Nobody knows but it was found in a book called Mercantile Arithmetic by Robert Recorde(16 votes)

- How would you represent negative numbers?(10 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.(6 votes)

- What person invented math(6 votes)
- No one person invented math, many people contributed to it's development over many, many years.(16 votes)

- how do i learn how to do more easy subtraction cuz i suck at subtraton and im super good at addition.(9 votes)
- how do I make adding easier(8 votes)
- you can write it all down on a piece of paper.(2 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.