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## Class 10 (Foundation)

### Course: Class 10 (Foundation)>Unit 1

Lesson 2: Comparing numbers

# Intro to direct & inverse variation

Sal explains what it means for quantities to vary directly or inversely, and gives many examples of both types of variation. Created by Sal Khan.

## Want to join the conversation?

• This might be a stupid question, but why do we use "k" as the constant?
(40 votes)
• I would imnagine it is from the German "konstant."
(10 votes)
• Does an inverse variation represent a line? Also, are these directly connected with functions and inverse functions?
(14 votes)
• How can π*x be direct variation? Pi is irrational, and keeps going on and on, so there would be no exact scale for both x and y. Thank you for the help!
(8 votes)
• The number pi is not going anywhere. It is fixed somewhere between 3 and 4. The y-scale could be indexed by pi itself.
(10 votes)
• Why is 4x + 3y = 24 an equation that does not represent direct variation?
(4 votes)
• The reason is that y doesn't vary by the same proportion that x does (because of the constant, 24).

In your equation, "y = -4x/3 + 6", for x = 1, 2, and 3, you get y = 4 2/3, 3 1/3, and 2. For x = -1, -2, and -3, y is 7 1/3, 8 2/3, and 10. Notice that as x doubles and triples, y does not do the same, because of the constant 6.

To quote zblakley from his answer here 5 years ago:
"The difference between the values of x and y is not what dictates whether the variation is direct or inverse. What is important is the factor by which they vary. While y becomes more negative as x becomes more positive, they will still vary by the same factor (i.e. if you increase x from 1 to 4 that's a factor of 4, the value of y [in y = -2x] will go from -2 (when x=1) to -8 (when x=4) which is also a factor of 4).
(8 votes)
• I don't get what varies means? Can someone tell me.
(2 votes)
• "Varies" refers to how one variable changes in relationship to the other variable.
(12 votes)
• I see comments about problems in a practice section. Besides the 3 questions about recognizing direct and inverse variations, are there practice problems anywhere?
(4 votes)
• At about , (when talking about direct variation) Sal says that "in general... if y varies directly with x... x varies directly with y." Are there any cases where this is not true? In other words, are there any cases when x does not vary directly with y, even when y varies directly with x?
Thanks!
(7 votes)
• i know this is a wierd question but what do you do when in a direct variation when your trying to find K what do you do when X wont go into Y evenly? do you just use decimal form or fraction form?
(3 votes)
• You can use the form that you prefer; the two are equivalent. In the Khan A. exercises, accepted answers are simplified fractions and decimal answers (except in some exercises specifically about fractions and decimals). If you're not sure of the format to use, click on the "Accepted formats" button at the top right corner of the answer box.
(5 votes)
• At , where you give the formula for inverse variation, I am confused. The formula that my teacher gave us was ( y = k/x ) Please help and thanks so much!!
(2 votes)
• Both your teacher's equation ( y = k / x) and Sal's equation ( y = k * 1/x) mean the same thing, like they will equal the same number. To show this, let's plug in some numbers.

How about x = 2 and k = 4?

Here's your teacher's equation:

``y = k / xy = 4 / 2y = 2``

and now Sal's:

``y = k * 1/xy = 4 * 1/2 (Since we know 1/2 equals .5, let's use that instead, usually people understand decimals better for multiplying, but it means the exact same as 1/2)y = 4 * .5(We are essentially taking half of 4)y = 2``

Would you like me to explain why? It takes a bit of explaining on fractions and how they work :)

------------------------------

If you want to see how we would multiply 4 * 1/2, here's a picture I drew to explain it =

http://i.imgur.com/EkViZOw.png
(5 votes)
• Why does a graph expressing direct proportionality always go through the origin?
(2 votes)
• Because in order for linear equation to not go through the origin, it has to be shifted i.e. have the form `y=mx+c` . But that will mean that x and y no longer vary directly (or inversely for that matter). For example, if `f(x)=x+2` , then:
`f(1) = 3` ; `f(2) = 4` , so while x increased by a factor of 2, f(x) increased by a factor of 4/3, which means they don't vary directly.
(5 votes)

## Video transcript

I want to talk a little bit about direct and inverse variations. So I'll do direct variation on the left over here. And I'll do inverse variation, or two variables that vary inversely, on the right-hand side over here. So a very simple definition for two variables that vary directly would be something like this. y varies directly with x if y is equal to some constant with x. So we could rewrite this in kind of English as y varies directly with x. And if this constant seems strange to you, just remember this could be literally any constant number. So let me give you a bunch of particular examples of y varying directly with x. You could have y is equal to x. Because in this situation, the constant is 1. We didn't even write it. We could write y is equal to 1x, then k is 1. We could write y is equal to 2x. We could write y is equal to 1/2 x. We could write y is equal to negative 2x. We are still varying directly. We could have y is equal to negative 1/2 x. We could have y is equal to pi times x. We could have y is equal to negative pi times x. I don't want to beat a dead horse now. I think you get the point. Any constant times x-- we are varying directly. And to understand this maybe a little bit more tangibly, let's think about what happens. And let's pick one of these scenarios. Well, I'll take a positive version and a negative version, just because it might not be completely intuitive. So let's take the version of y is equal to 2x, and let's explore why we say they vary directly with each other. So let's pick a couple of values for x and see what the resulting y value would have to be. So if x is equal to 1, then y is 2 times 1, or is 2. If x is equal to 2, then y is 2 times 2, which is going to be equal to 4. So when we doubled x, when we went from 1 to 2-- so we doubled x-- the same thing happened to y. We doubled y. So that's what it means when something varies directly. If we scale x up by a certain amount, we're going to scale up y by the same amount. If we scale down x by some amount, we would scale down y by the same amount. And just to show you it works with all of these, let's try the situation with y is equal to negative 2x. I'll do it in magenta. y is equal to negative-- well, let me do a new example that I haven't even written here. Let's try y is equal to negative 3x. So once again, let me do my x and my y. When x is equal to 1, y is equal to negative 3 times 1, which is negative 3. When x is equal to 2, so negative 3 times 2 is negative 6. So notice, we multiplied. So if we scaled-- let me do that in that same green color. If we scale up x by 2-- it's a different green color, but it serves the purpose-- we're also scaling up y by 2. To go from 1 to 2, you multiply it by 2. To go from negative 3 to negative 6, you're also multiplying by 2. So we grew by the same scaling factor. And if you wanted to go the other way-- let's try, I don't know, let's go to x is 1/3. If x is 1/3, then y is going to be-- negative 3 times 1/3 is negative 1. So notice, to go from 1 to 1/3, we divide by 3. To go from negative 3 to negative 1, we also divide by 3. We also scale down by a factor of 3. So whatever direction you scale x in, you're going to have the same scaling direction as y. That's what it means to vary directly. Now, it's not always so clear. Sometimes it will be obfuscated. So let's take this example right over here. y is equal to negative 3x. And I'm saving this real estate for inverse variation in a second. You could write it like this, or you could algebraically manipulate it. You could maybe divide both sides of this equation by x, and then you would get y/x is equal to negative 3. Or maybe you divide both sides by x, and then you divide both sides by y. So from this, so if you divide both sides by y now, you could get 1/x is equal to negative 3 times 1/y. These three statements, these three equations, are all saying the same thing. So sometimes the direct variation isn't quite in your face. But if you do this, what I did right here with any of these, you will get the exact same result. Or you could just try to manipulate it back to this form over here. And there's other ways we could do it. We could divide both sides of this equation by negative 3. And then you would get negative 1/3 y is equal to x. And now, this is kind of an interesting case here because here, this is x varies directly with y. Or we could say x is equal to some k times y. And in general, that's true. If y varies directly with x, then we can also say that x varies directly with y. It's not going to be the same constant. It's going to be essentially the inverse of that constant, but they're still directly varying. Now with that said, so much said, about direct variation, let's explore inverse variation a little bit. Inverse variation-- the general form, if we use the same variables. And it always doesn't have to be y and x. It could be an a and a b. It could be a m and an n. If I said m varies directly with n, we would say m is equal to some constant times n. Now let's do inverse variation. So if I did it with y's and x's, this would be y is equal to some constant times 1/x. So instead of being some constant times x, it's some constant times 1/x. So let me draw you a bunch of examples. It could be y is equal to 1/x. It could be y is equal to 2 times 1/x, which is clearly the same thing as 2/x. It could be y is equal to 1/3 times 1/x, which is the same thing as 1 over 3x. it could be y is equal to negative 2 over x. And let's explore this, the inverse variation, the same way that we explored the direct variation. So let's pick-- I don't know/ let's pick y is equal to 2/x. And let me do that same table over here. So I have my table. I have my x values and my y values. If x is 1, then y is 2. If x is 2, then 2 divided by 2 is 1. So if you multiply x by 2, if you scale it up by a factor of 2, what happens to y? y gets scaled down by a factor of 2. You're dividing by 2 now. Notice the difference. Here, however we scaled x, we scaled up y by the same amount. Now, if we scale up x by a factor, when we have inverse variation, we're scaling down y by that same. So that's where the inverse is coming from. And we could go the other way. If we made x is equal to 1/2. So if we were to scale down x, we're going to see that it's going to scale up y. Because 2 divided by 1/2 is 4. So here we are scaling up y. So they're going to do the opposite things. They vary inversely. And you could try it with the negative version of it, as well. So here we're multiplying by 2. And once again, it's not always neatly written for you like this. It can be rearranged in a bunch of different ways. But it will still be inverse variation as long as they're algebraically equivalent. So you can multiply both sides of this equation right here by x. And you would get xy is equal to 2. This is also inverse variation. You would get this exact same table over here. You could divide both sides of this equation by y. And you could get x is equal to 2/y, which is also the same thing as 2 times 1/y. So notice, y varies inversely with x. And you could just manipulate this algebraically to show that x varies inversely with y. So y varies inversely with x. This is the same thing as saying-- and we just showed it over here with a particular example-- that x varies inversely with y. And there's other things. We could take this and divide both sides by 2. And you would get y/2 is equal to 1/x. There's all sorts of crazy things. And so in general, if you see an expression that relates to variables, and they say, do they vary inversely or directly or maybe neither? You could either try to do a table like this. If you scale up x by a certain amount and y gets scaled up by the same amount, then it's direct variation. If you scale up x by some-- and you might want to try a couple different times-- and you scale down y, you do the opposite with y, then it's probably inverse variation. A surefire way of knowing what you're dealing with is to actually algebraically manipulate the equation so it gets back to either this form, which would tell you that it's inverse variation, or this form, which would tell you that it is direct variation.