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## Early math review

### Course: Early math review > Unit 2

Lesson 5: Relate addition and subtraction# Relating addition and subtraction

Sal shows the relationship between 2+3=5, 5-2=3, and 5-3=2.

## Want to join the conversation?

- 2 green balls and 3 purple balls is 5 balls together(16 votes)
- that is correct mfunk0886. Good job for getting that correct.(2 votes)

- There are 4 types of basic adding, subtracting.( I think ). There is addition+, subtraction-,multiplicationX, and dividing/. Now, if you are willing to add, you need to take the first number and add that to the second number. Then you have your answer. Subtraction is different, I you have a number, and a different number than you have got to take away the how ever many numbers from the first number of the second number. Now, multiplication is a whole new level. The way you work this type is to add the first number however many times the second number is. Dividing is harder. You have the first number and the lower second number. You have got to put in the second number until you have the first. You count however many times you put in the second number until you have the remaining first number. Than you have the answer! Your welcome! (I hope this helps)(19 votes)
- Um what is a reciprocal(5 votes)
- Good question for first grade!

A reciprocal of a number is 1 divided by that number. Taking a reciprocal can also be thought of as switching the numerator (top) and denominator (bottom) of a fraction.

Examples:

Because 1 is 1/1, the reciprocal of 1 is just 1.

Since 3 is 3/1, the reciprocal of 3 is 1/3.

The reciprocal of 4/7 is 7/4, or 1 3/4.

Because 2 4/5 is 14/5, the reciprocal of 2 4/5 is 5/14.

0 is the only number without a reciprocal since 1/0 is undefined.

Note that any number (other than 0) times its reciprocal is always 1. Later on, when you study operations with fractions, you will learn that division by a number is the same as multiplication by that number's reciprocal.(6 votes)

- Just by looking at the addition problem we can also understand the subtraction problem without writing the subtraction problem.(7 votes)
- Math goes on infinitely, does that mean it goes infinitely both positively and negatively? So, if it is limitless in positive numbers (above zero), then would the opposite of infinity be 'negative infinity' or simply, again, infinity?(4 votes)
- 101010101001010101010100101(3 votes)
- i need help with my math. {PLZ HELP!}(4 votes)
- i ts easy 1,342+1,327=2,669(3 votes)
- What does relation mean in this instance?(3 votes)
- Relating means showing how they are connected. Subtraction is the inverse or opposite of addition. For example 3 - 1 =2 and 2 + 1 = 3.(1 vote)

- Why was the plus symbol chosen as the plus symbol and the minus?(2 votes)
- Because it creates an easy way to know what to do when doing math(1 vote)

## Video transcript

I have two green balls
and three purple balls so how many balls
do I have altogether? So two green balls
and three purple balls is going to be 5 balls altogether. We can count them;
one, two, three, four, five. Now what if we did something
a little different? Let's start with the five balls,
and let's take away two. Let's subtract two of these balls. Take away one and two,
and what are we left with? We are left with
one, two, three balls. We're left with three balls. You might see a pattern here;
two plus three is equal to five but if you start with five
and take away two, you're going to be left with three. Look how they're arranged. Two plus three is equal to five
and five minus two is three. It works the other way around. If I start with five
and I were to take away three, if I subtract one, two, three,
what am I left with? I'm left with one, two,
I'm left with two. So two plus three is five,
three plus two is five, which is another way of saying
that five minus two is three, or five minus three is two. Take a look at this, it's a really important
idea to understand in math. If you add two numbers
and you get another number, then you take the result
and subtract one of these you're going to get the other number.