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### Course: Get ready for Algebra 1 > Unit 3

Lesson 1: Constant of proportionality- Introduction to proportional relationships
- Identifying constant of proportionality graphically
- Constant of proportionality from graph
- Constant of proportionality from graphs
- Identifying the constant of proportionality from equation
- Constant of proportionality from equation
- Constant of proportionality from equations
- Constant of proportionality from tables
- Constant of proportionality from tables
- Constant of proportionality from table (with equations)
- Constant of proportionality from tables (with equations)

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# Introduction to proportional relationships

Proportional relationships are relationships between two variables where their ratios are equivalent. Another way to think about them is that, in a proportional relationship, one variable is always a constant value times the other. That constant is known as the "constant of proportionality".

## Want to join the conversation?

- i need more help with this, i don't understand(33 votes)
- Well, a proportional relationship means that the ratio between two variables stayed the same.

Eg.

4 eggs = 2 cups of milk

8 eggs = 4 cups of milk

30 eggs = 15 cups of milk

As you can see, this is a proportional relationship because the ratio between the number of egg and cup of milk is 2:1 through out the table.

A non-proportional relationship os when the two variables have different ratio

Eg.

4 eggs = 4 cups of milk

5 eggs = 6 cups of milk

13 eggs = 12 cups of milk

This is not a proportional relationship because there is no same ratio in the table.

Hope that help.(48 votes)

- why are the things that you are teaching us easier than the questions that khan academy is asking us?(25 votes)
- Actually, some of these problems, you may find in the Khan Academy curriculum. Sal just explains the question expertly.(17 votes)

- is it just me ...or we are all staring at our screens like ..."ummm..don't get it"?(23 votes)
- Why do teachers force us to do Khan Academy?(21 votes)
- aperently it gives teachers ahead on the leson(3 votes)

- im crying inside(13 votes)
- maths would be so much harder without this(12 votes)
- If you feel math is too hard look at these jokes to lighten you down:

1. Why was the math book sad? Because it had too many proportion-al relationships!

2. How do you make a proportional relationship laugh? You tell it a funny percentage!

3. I used to have a proportional relationship, but then it just didn't measure up.

4. Why did the proportional relationship break up? It couldn't maintain a steady connection.

5. I told my friend a joke about proportional relationships, but he didn't understand it. It just didn't scale well with him.

6. I asked my math teacher for a joke about proportional relationships, and he said, "I can't, I'm not in the right ratio of humor at the moment."

7. My friend tried to make a joke about proportional relationships, but it was a fraction of what I expected.

8. The proportional relationship and the straight line had an argument, but they resolved it fairly.

9. Why did the proportional relationship go to therapy? It needed help finding its balance.

10. I tried to write a joke about proportional relationships, but it just didn't add up.(11 votes)- Thank you for making me laugh today, these jokes made it a little bit brighter!(2 votes)

- it's not that annoying, it's pretty cool!(9 votes)
- Hello, I am new. If any of you are new, and you don't understand, it means that it has to be divided by the first number. Ex. (2:4, or, 7;14, or even, 12;24, as he said in the video. If I am wrong, please make a better example of what I was trying to say. Bye!(9 votes)

## Video transcript

- [Instructor] In this video, we are going to talk about proportional relationships, and these are relationships between two variables where the ratio between the variables is equivalent. Now that sounds complex
or a little bit fancy. It'll hopefully seem a little bit more straightforward once
we look at some examples. So let's say I'm looking at a recipe for some type of baked goods, maybe it's some type of pancakes, I've been making a lot of those lately, and we know that for a
certain number of eggs how many cups of milk we need. So we have number of eggs, and then we're also going
to have cups of milk. And in this recipe, we know that if we're going to use one egg, then we would use two cups of milk, and if we use three eggs, then we're gonna use six cups of milk, and if we use, let's say, 12 eggs, then we're going to use 24 cups of milk. So is this a proportional relationship where the two variables
are the cups of milk and the number of eggs? Well, to test that we just have to think about the ratio between these two variables. And you can say that the ratio of the number of eggs to the cups of milk, or the ratio of the cups of milk to the number of eggs. But you just need to ensure that they are always equivalent in these scenarios. So let me make another column here, and I'm gonna think about the ratio of the eggs to the cups of milk. Well, in this first scenario one egg for two cups of milk. This second scenario is three to six. This third scenario is 12 to 24. Are these equivalent ratios? Well, to go from one to three you multiply by three, and also, to go from two to six, you multiply by three. So you multiplied both
the variables by three. Similarly, if you multiply the number of eggs by four, then you multiply the number of cups of milk by four as well. So these indeed are all equivalent ratios, one to two, three to six, 12 to 24. In every scenario you have twice as much cups of milk as you have number of eggs. So this would be proportional. So check. Now what would be an example of a non-proportional relationship? We'll stay in this baked
goods frame of mind. Let's say you're going to a cake store and you're curious about how much it would cost to buy a cake for different numbers of people. So let's say number of servings, number of servings in one column, and then the cost of the cake. And let me set up two
columns right over here. And so let's say if you have 10 servings, the cake costs $20. If you have 20 servings,
the cake costs $30. And if you have 40 servings,
the cake costs $40. Pause this video and see
if you can figure out whether this is a
proportional relationship. If it is, why? If it isn't, why not? Alright, well let's just think about the ratios again. And here our two variables are the number of servings
and the cost of cake. So if we look at the
ratio of the servings, servings, to cost, in this first situation it is 10 to 20, and then it is 20 to 30, and then it is 40 to 40. And so to see if these
are equivalent ratios, when we go from 10 to 20 on the number of servings, we're multiplying by two. But when we go from 20 to 30 on the cost of the cake, we aren't multiplying by two, we're multiplying by
1.5, or one and a half. And similarly, when we go from 20 to 40, we are multiplying by two again, but to go from 30 to 40 we aren't multiplying by two, we're multiplying by one and one third. By one and one third. When we multiply our
servings by a given amount, we're not multiplying our cost of cake by the same amount. This tells us that this
is not proportional. One way to think about
proportional relationships, we already said, that the ratio between the variables will be equivalent. Another way to think about it is one variable will always be some constant times the first variable. So in our first example right over here we said the cups of milk is always two times the number of eggs. We can write that down. So cups of milk is always going to be equal to two times the number of eggs. And this number right over here, we call that the constant
of proportionality. And you wouldn't be able to set up an equation like this in this scenario. It would have to be more complicated. And so a proportional relationship, the ratios are equivalent between the two variables and you can set it up with an equation like this where you have a constant of proportionality.