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Constant of proportionality from equation

Sal identifies the constant of proportionality from equations.

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  • starky sapling style avatar for user madeline
    does anyone else not get this 😌🙃
    (32 votes)
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  • duskpin ultimate style avatar for user craycray_unicorn
    Is there a specific reason that you always solve for y, and not x?
    (18 votes)
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  • starky tree style avatar for user 1zbake1891
    how are you able to tell 4 is not 1/2 if it only says 4/x , the x could =8?
    (9 votes)
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    • aqualine ultimate style avatar for user ScienceLover7
      y = 4/x
      What this equation tells you is that for whatever value of x, y will equal 4 divided by that value.
      Here, x and y are variables, their value changes.
      This means that the y-value depends on the x-value. They need not be fixed : x and y can equal anything, but their product will always be four.
      The constant of proportionality here is the 4, because it is the only thing which has a fixed value.

      P.S. : This is a different type of proportion, called inverse proportion. Here, as one variable increases, the other decreases and vice-versa. The equation is in the form of y = k*1/x.
      Hope this helps :)
      (0 votes)
  • duskpin sapling style avatar for user Hana Hussain
    at how did you get 1/2 dont u do 6 divided by 3 is 2? im a little confused?
    (1 vote)
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  • duskpin ultimate style avatar for user McBoi
    If there's a constant of proportionality between x and y, and you make a graph to show it, would the constant of proportionality be the same thing as the slope of the line?
    (5 votes)
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  • piceratops ultimate style avatar for user Lucie Peterson
    Sometimes I get my variables mixed up and end up thinking the constant of proportionality/unit rate of change/slope is, for example, 3 when it is supposed to be 1/3. when there is an x and a y I know what to do because I remember xk=y, but when I've got a word problem where the variables are z and h or n and j, I get all mixed up again and before I know it I've got hours per mile instead of miles per hour! Any tips on how to remember which one is which?
    (3 votes)
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    • stelly blue style avatar for user Kim Seidel
      You may find it helpful to keep track of the units of measurement your are working with. Label the numbers with miles and hours so you can see your result in units. If the problem asks your for miles/hour, then you know you need to divide the miles by the number of hours.

      Hope this helps.
      (5 votes)
  • aqualine seedling style avatar for user waylenwomack
    why do we do these
    (3 votes)
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  • cacteye green style avatar for user badon0518
    Question one doesn't make sense to me,

    So we have

    4y = 8x

    so for Y=KX We find what times X = Y or Y/X. 4/8 (Y/X) not 8/4 (X/Y)

    So Y=1/2 . 8X = 4,

    but Sal wrote Y=2X 2 . 8X = 16 not our Y = 4
    (2 votes)
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  • starky sapling style avatar for user Lukas Binder
    why do people like math?
    why did he use y=kx?
    (3 votes)
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  • blobby blue style avatar for user Luke Greathouse
    please help me i know nothing
    (3 votes)
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Video transcript

- [Instructor] We are asked, "What is the constant "of proportionality in the equation 4y is equal to 8x?" Pause this video and have a go at this question. All right, so we might be used to seeing constants of proportionality when we have equations in a slightly different form. A constant of proportionality is what do you multiply x by to get to y? So y would be equal to our constant of proportionality times x. But this isn't written in that form, so what we do is manipulate it a little bit so that we can see it in that form. And the obvious thing is we just need to solve for y. So right now it says 4y is equal to 8x. Well, if we wanna solve for y, we can just divide both sides by four, and we are left with y is equal to eight divided by four, which is two times x. Well, now the constant of proportionality jumps out at us. To get y, we have multiply x by two. That is our constant of proportionality. Let's do another example. Here we're asked, "Which equation has a constant "of proportionality equal to 1/2?" Again, pause the video. Try to answer it yourself. Okay, so I'm just gonna go equation by equation and calculate their constants of proportionality and see which one has a constant of proportionality equal to 1/2. So this one right over here, choice A clearly has a constant of proportionality of 1/8, so we can just rule that out. Equation B right over here clearly has a constant of proportionality of four, not 1/2, so we can rule that one out. Let's see, the constant of proportionality for equation C, if we wanna solve for y, we could divide both sides by six. And so we're gonna get y is equal to 3/6 times x. Well, 3/6 is the same thing as 1/2 times x, and so there you have it. We have a constant of proportionality of 1/2. That's the choice I like. And we can verify that this one doesn't work. If you wanna solve for y, you divide both sides by three, and you get y is equal to nine divided by three is 3x, so here our constant of proportionality is three, so we can feel good about choice C.