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### Course: Algebra (all content) > Unit 13

Lesson 7: Direct and inverse variation- Intro to direct & inverse variation
- Recognizing direct & inverse variation
- Recognize direct & inverse variation
- Recognizing direct & inverse variation: table
- Direct variation word problem: filling gas
- Direct variation word problem: space travel
- Inverse variation word problem: string vibration
- Proportionality constant for direct variation

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# Recognizing direct & inverse variation: table

Sal is given a table with a few values of the variables x and y, and determines whether the variables vary directly or inversely. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- at about3:15in the video. Can someone explain the difference between direct and joint? thanks(25 votes)
- Well...direct variation is y=kx, whereas in joint, it is y=kzx(6 votes)

- How are these concepts of direct, inverse, and joint variation used in everyday life?(4 votes)
- Direct variation occurs all the time - whenever you have item pricing. If macadamias are $8 per pound, then cost and quantity of food are in a direct relationship. The more macadamias you want, the more you have to spend.
`cost = 8 * pounds`

.

Inverse relationships come up whenever you're splitting something. If you bought 10 pounds of macadamia nuts, and have unknown number of people coming over to eat it, the serving size that person eats is inverse to the number of people that come over.`serving = 10 / people`

.

Joint variation is a combination of the two concepts. Instead of`serving = 10 / people`

, you could write`serving = pounds / people`

, where you've created a variable for pounds of food instead of it being a constant.(16 votes)

- Is there a video about joint variation? Because it would really help! Thanks.(10 votes)
- Sal said that there is a video on joint variation in the Introductory Algebra Course.(1 vote)

- do you have a video on PARTIAL variation??

thanks(6 votes) - Sal said that the direct relation means that when x increases y will also increase, but what if the constant is negative? It becomes inverse right?(4 votes)
- If the constant is negative and you increase x, then y will become more negative, so it's still a direct variation.(2 votes)

- if w varies directly with x, x varies inwersely with y, and y varies inversely with z what equation would show how w relates to z?(2 votes)
- my answer is w=k(zbc),

to get this i started with three equations w=kx, x=yc, and y=zb (note c and b are just versions of k) then i preformed subsitution until i got to one equation, hope this helps(5 votes)

- Does direct variation automatically indicate a linear function?(2 votes)
- Yes. Here is why:

The equation of a linear function is y=mx+b.

If b=0, then you get y=mx, the formula for direct variation.(3 votes)

- Is it conventional to write y= kx or x=ky?(2 votes)
- The convention is to write the dependent variable (the vertical axis) on the left by itself with all the terms containing the independent variable (the horizontal axis) on the right.

So, if y were a function of x, then x would be the independent variable and y would be the dependent variable. i.e. y = kx(3 votes)

- Could someone please explain2:47in the video(3 votes)
- So based on the video, if you triple "x," then you multiply y by the reciprocal, which is 1/3.(1 vote)

- How can you tell whether a set of data pairs (x,y) show inverse variation?(2 votes)
- inverse variation:

Two variables x and y show if they are related as follows:

y = kx, k ≠ 0

The nonzero constant k is called the constant of variation and y is said to vary

inversely with x.

this is what i know hope it helps!(2 votes)

## Video transcript

Determine whether the data in the table is an example of direct inverse or joint
variation. Then identify the equation that represents
the relationship. So let's just think about what direct
inverse or joint variation even means. So if you have direct variation. Direct variation. So if y varied directly with x it
literally means that y is equal to some constant multiple of x, or if you divide
both sides of this by x it means that y over x is equal to k so the ration
between y and x is a constant. And you could go the other way around. You could also say that x is equal to some constant not, not going to be the same
constant times y. Or that x over y is going to be equal to
some other constant. So these aren't necessarily the same k. All I'm just saying is that it's a
constant relationship. These are all examples of direct
variation. In dir, or I should say inverse variation
is to some degree the opposite depending on how
you view the opposite. And before I even talk about that, let's think about the telltale signs of direct
variation. If x increases, y should increase. So if x increases. Let me do that in the same yellow. So the telltale signs of direct variation,
if x increases then y will increase and vice
versa. The other telltale sign is. Is if you increase x by some, by some
factor. So, if you have x going to 3x then y
should also increase by that same factor. And we could see that with some examples. So, I mean, you could pick a K, let's say
that, let's say that K was one. So if y is equal to x, if you take, if x
goes from one to three, then y is also going to go
from one to three. So that's all we're talking about here. Let me actually, y should actually to three times y, that's what I'm talking
about. If you triple x, you're also gonna end up
tripling y. Inverse variation. You have y being equal to some constant
times one over x. So instead of an x here you have a one
over x or if you multiply both sides by x you get x times y
is equal to some constant. And you could switch the x's and the y's
around as well for inverse variation. Now what are the tale tale signs? Well if you increase x, if x goes up, then
what happens to y? If x goes up then this becomes a smaller
value cuz it's one over x so then y will go
down. Then y will go down. And if you take X and if you're to say
increase it by a factor of three then what's going to
happen to Y? Well if you increase this by a factor of three, you're actually going to decrease
this whole value by a factor of one-third, so Y is going to go, so then you're going to have one-third
of y. So that's, these are the tell-tale signs
for inverse variation. Now finally they talk about something
called joint variation, and this one you won't necessarily see in
introductory algebra course. But joint variation deals with more than
one variable. So if I told you, if I told you that area
of a rectangle is equal to the width of a rectangle times the length of rectangle,
this is an example of joint variation. Area is proportionally to two. Is the proportional to two different
quantities? So the main tell tale sign here for joint
variation frankly is you're gonna be dealing with
more than two variables. Joint, Joint, Joint variation. So when you look at this example, there
only gi, giving us two variables. So you can rule out joint variation just
right from the get go. Now let's look at the tell tale signs. So as x is increasing, as x goes from one
to two, what is happening to y? Y went from 12 to six. So as x is going up by a factor of two, y is going, is, is going by a factor of
one half. Or y is being multiplied by one half. So as x goes from one to three, it's being
multiplied by three. Y is being multiplied or I guess could say
is, is multiplied by one-third. So it's definitely not direct variation. As x increases, y is decreasing. So it's definitely not direct variation. And then really you can just rule out,
since we rule out the other two, you can probably guess this is
going to be inverse variation. But we can validate it. When X increases, Y is decreasing. When X increases by a certain factor, Y is increasing by one over that factor, which
is actually decreasing. So, if you go from one to three, if X is
being multiplied by three, then Y essentially
becomes one third of its original value. When X is one Y is 12 when X is three Y is
four so we have inverse variation in play. Now they ask us identify the equation that
represents the relationship. Well we know with inverse variation the
product of x and y need to be equal to some constant. So that if we take x times y over here so
lets just multiply lets make another column here
call this the x times y column. One times 12 is 12. Two times six is 12. Three times four is 12. Four times three is 12. So, clearly in every situation, x times y
is, is a constant and it is 12. So, the equation that represents the
relationship, it is, X, Y is equal to 12 and that is clearly an
inverse