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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?
• Below zero there would be negative numbers
• How far does negative numbers go? Is it like normal numbers? Never ending?
• Yes, Negative numbers go on forever just on the other side of the number line.
• Why subtraction has this (-) sign?
• It is easier to use the (-) sign is stead of saying plus a negative.
• How would you represent negative numbers?
• 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.
• How do you write subtraction of negative numbers?? Like this: -2 - -1? It looks kinda weird to me.
• 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.
• what if you did 4-5? how would that even be possible? would it be 1?
• 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.
• 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?
• 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!
• who made subtraction? as like: who create the subtraction
• 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.
• I don't understand anything! So what is 3-1?
• 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