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### Course: 8th grade foundations (Eureka Math/EngageNY)>Unit 5

Lesson 1: Topic A: Foundations

# Intro to proportional relationships

To know if a relationship is proportional, you should look at the ratios between the two variables. If the ratio is always the same, the relationship is proportional. If the ratio changes, the relationship is not proportional.

## Want to join the conversation?

• What if an odd and even number were in a proportional.
• If an odd and an even were in a proportional like 2:3 it would just be like 4:6
• What is 23%of 80
• (1.) Turn 23% into the decimal .23.
(2.) Multiply .23 and 80.
(3.) .23 X 80 = 18.4
• This is great.
I've been doing this for about an hour and it's growing on me. I'd like to give a big thanks to the guy who makes these.
• I totally agree
• Can you make this more easier like more understandable?
• Proportions are the same ratios written in different forms. A proportional relationship is states that they are the same. For example, 1/2 and 6/12 have a proportional relationship, which means they are the same.

I hope this helps!!
• Can I add a fraction
• Yes as long as they have the same denominator
(1 vote)
• why do we do this very boring
• So you can get smarter
• What does constant of proportionality mean and why does it matter?
• It is the same thing as slope of a line IF the line goes through the origin (0,0). y=kx is the formula or k = y/x. Note that the general slope formula, m = (y2-y1)/(x2-x1) can be used with (0,0) to get m = (y2-0)/(x2-0) or m = k = y2/x2. It is always the same no matter which point on the line you choose, thus constant.
• What is a rate?
• A rate is essentially a constant. This constant cannot and must not change. Like Jada posed the example, I will take it one step further: A car is going at 25 miles per five hours, find the unit rate; the unit rate is actually just someone compared to one. So for every one of that object, there are x number of the other
• this is boring
• About 4 years ago the comment section was filled with useful questions related to the video, but now all these comments are usless
• How do you write a equation that describes the relationship

## Video transcript

What I want to introduce you to in this video is the notion of a proportional relationship. And a proportional relationship between two variables is just a relationship where the ratio between the two variables is always going to be the same thing. So let's look at an example of that. So let's just say that we want to think about the relationship between x and y. And let's say that when x is one, y is three, and then when x is two, y is six. And when x is nine, y is 27. Now this is a proportional relationship. Why is that? Because the ratio between y and x is always the same thing. And actually the ratio between y and x or, you could say the ratio between x and y, is always the same thing. So, for example-- if we say the ratio y over x-- this is always equal to-- it could be three over one, which is just three. It could be six over two, which is also just three. It could be 27 over nine, which is also just three. So you see that y over x is always going to be equal to three, or at least in this table right over here. And so, or at least based on the data points we have just seen. So based on this, it looks like that we have a proportional relationship between y and x. So this one right over here is proportional. So given that, what's an example of relationships that are not proportional. Well those are fairly easy to construct. So let's say we had-- I'll do it with two different variables. So let's say we have a and b. And let's say when a is one, b is three. And when a is two, b is six. And when a is 10, b is 35. So here-- you might say look, look when a is one, b is three so the ratio b to a-- you could say b to a-- you could say well when b is three, a is one. Or when a is one, b is three. So three to one. And that's also the case when b is six, a is two. Or when a is two, b is six. So it's six to two. So these ratios seem to be the same. They're both three. But then all of sudden the ratio is different right over here. This is not equal to 35 over 10. So this is not a proportional relationship. In order to be proportional the ratio between the two variables always has to be the same. So this right over here-- This is not proportional. Not proportional. So the key in identifying a proportional relationship is look at the different values that the variables take on when one variable is one value, and then what is the other variable become? And then take the ratio between them. Here we took the ratio y to x, and you see y to x, or y divided by x-- the ratio of y to x is always going to be the same here so this is proportional. And you could actually gone the other way. You could have said, well what's the ratio of x to y? Well over here it would be one to three, which is the same thing as two to six, which is the same thing as nine to 27. When you take this ratio-- if you say the ratio of x to y instead of y to x, you see that it is always one third. But any way you look at it-- the ratio between these two variables-- if you say y to x, it's always going to be three. Or x to y is always going to be one third. So this is proportional while this one is not.