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### Course: Arithmetic (all content)>Unit 2

Lesson 1: Basic addition and subtraction

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

## Want to join the conversation?

• What does relation mean in this instance?
• Relating means showing how they are connected. Subtraction is the inverse or opposite of addition. For example 3 - 1 =2 and 2 + 1 = 3.
• If 3+2=5 & 5-3=2, would we call this a relationship? Does this work for everything? Eg. x+y=z & z-y=x?
• Yes, it is a relationship and will work for everything. Try to plug in numbers and test for yourself.
• What other relations exist in math? Aside from this (adding vs. subtracting) and multiplication vs. division (as Marvin mentions in his question above). Are those the only two?
• there are numerous relations in math, but they may not appear until later on. The term you're likely referring to is an inverse function. These are very important in higher level math, likely starting with trigonometry and becoming even more important to Calculus.

## 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.