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Equations for proportional relationships

Learn how to write a proportional equation y=kx where k is the so-called "constant of proportionality".

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  • starky seed style avatar for user angelramirez-jimenez
    i dont understand this
    (8 votes)
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  • blobby green style avatar for user dr3862
    Never gonna give you up
    Never gonna let you down
    Never gonna run around and desert you
    Never gonna make you cry
    Never gonna say goodbye
    Never gonna tell a lie and hurt you
    (3 votes)
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  • piceratops tree style avatar for user Sean Cancel
    what is real life example of the equation y=1/20*x
    (4 votes)
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  • blobby green style avatar for user Jannie Wick
    the video doesn't show how to get a decimal from the graph
    (4 votes)
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  • female robot ada style avatar for user Haley Fellows
    So I am doing the practice problems for this right now and sometimes the constant of proportionality is a fraction, like "y=1/3x" but sometimes it is a number or a decimal, like "y=0.34x" or "y=4x". How do I know which one to do? There have been multiple times where I put the decimal equal to the fraction, like 0.33 for 1/3 and gotten it wrong because it was supposed to be the fraction (and vice versa)
    (2 votes)
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    • aqualine ultimate style avatar for user Michelle Faler
      I have experienced similar issues entering answers. After doing quite a few of these types of problems, I have found that entering your answer as a fraction is the safer bet, especially when your answer is something like 10/7x=y as 10/7 is a repeating decimal. I enter the answer as a decimal only if the question prompts me to do something like "round my answer to the nearest hundredth." Then I obviously know my answer should have a decimal in it. Otherwise, I just seem to run into problems with entering them usually do to rounding.

      So yeah, I just feel that it is better to answer these kinds of questions with a fraction instead of the decimal unless you are specifically told to do so in the question.
      (2 votes)
  • blobby green style avatar for user Superglue1339
    ?? does this mean in real life ?_?
    (3 votes)
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    • mr pink green style avatar for user David Severin
      What about buying things at the grocery store. A can of beans costs .99. This gives a proportional relationship cost (c) and number of cans (n) gives a formula c=.99n. This allows you to tell you cost depending on how many you buy. If you are the store buying a large quantity of beans, there is probably lower costs per unit depending on the number of cans you buy (in the hundreds or thousands of cans). You can figure out your maximum profit based on buying the correct number of cans, you want to get close to the number you project to sell because if you buy too many and do not sell them all, you may have to discard them when they expire, so while you bought them cheaper, it may not be to your best advantage.
      (2 votes)
  • starky sapling style avatar for user DelilahH
    i have a question, how is 8/2 and 12/3 equal to 4
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  • mr pants teal style avatar for user benjaminjacobson
    This video taught me what my math teacher has been trying to teach my class for half a month now in the span of 1 minute and 18 seconds. Thanks Khan Academy.
    (3 votes)
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  • blobby green style avatar for user amand1201
    how to turn a slope-intercept equation into a proportional equation
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  • blobby green style avatar for user 28tgodfrey
    what a;jfafjjkl;adf;kljelkjaklj;afjk;
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Video transcript

- So, let's set up a relationship between the variables x and y. So, let's say, so this is x and this is y, and when x is one, y is four, and when x is two, y is eight, and when x is three, y is 12. Now, you might immediately recognize that this is a proportional relationship. And remember, in order for it to be a proportional relationship, the ratio between the two variables is always constant. So, for example, if I look at y over x here, we see that y over x, here it's four over one, which is just four. Eight over two is just four. Eight halves is the same thing as four. 12 over three it's the same thing as four. Y over x is always equal to four. In fact, I can make another column here. I can make another column here where I have y over x, here it's four over one, which is equal to four. Here it's eight over two, which is equal to four. Here it's 12 over three, which is equal to four. And so, you can actually use this information, the ratio, the ratio between y and x is this constant four, to express the relationship between y and x as an equation. In fact, in some ways this is, or in a lot of ways, this is already an equation, but I can make it a little bit clearer, if I multiply both sides by x. If I multiply both sides by x, if I multiply both sides by x, I am left with, well, x divided by x, you'd just have y on the left hand side. Y is equal to 4x and you see that's the case. X is one, four times that is four. X is two, four times that is eight. So, here you go, we're multiplying by four. We are multiplying by four, we are multiplying by four. And so, four, in this case, four, in this case, in this situation, this is our constant of proportionality. Constant, constant, sometimes people will say proportionality constant. Constant of proportionality, portionality. Now sometimes, it might even be described as a rate of change and you're like well, Sal, how is this a, how would four be a rate of change? And, to make that a little bit clearer, let me actually do another example, but this time, I'll actually put some units there. So let's say that, let's say that I have, let's say that x-- Let me do this, I already used yellow, let me use blue. So let's x, let's say that's a measure of time and y is a measure of distance. Or, let me put it this way, x is time in terms of seconds. Let me write it this way. So, x, x is going to be in seconds and then, y is going to be in meters. So, this is meters, the units, and this right over here is seconds. So, after one second, we have traveled, oh, I don't know, seven meters. After two seconds, we've traveled 14 meters. After three seconds, we've traveled 21 meters, and you can verify that this is a proportional relationship. The ratio between y and x is always seven. Seven over one, 14 over two, 21 over three. But, I wanna write that in terms of it's units. So, y over x is going to be, if we look at this point right over here, it's seven meters over one second. Seven meters over one second, or it's equal to seven meters per second. If you look at it right over here, if you say y over x, it's 14 meters, 14 meters, in two seconds, in two seconds. Well, 14 over two is seven, 14 over two is seven, and then the units are meters per second. So, that's why this constant, this seven, in all of these cases we have y over x is equal to seven, that this is also sometimes considered a rate. And over here it's very clear, this is my distance per time. Now, if I wanted to write it generally, I could say that, look, if I'm dealing with a proportional relationship, it's going to be of the form, I can always construct and equation of the form, of the form, either y over x is equal to k, where k is some constant. In this first example, k was equal to four and in this second example, k is equal to seven. Or, you can just manipulate it algebraically, multiply both sides by x and you would have y is equal to, y is equal to kx, where once again k is our constant of proportionality or proportionality constant. So, this is a really, in some ways it's a very simple idea, but in a lot of ways, you'll see this showing up multiple, many, many times in your mathematical career and it's neat to be able to recognize this as a proportional relationship.