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# Constant of proportionality from equation

Sal identifies the constant of proportionality from equations.

## Want to join the conversation?

• does anyone else not get this 😌🙃
• Is there a specific reason that you always solve for y, and not x?
• You normally solve the constant of proportionality for first number to second number.
• how are you able to tell 4 is not 1/2 if it only says 4/x , the x could =8?
• 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 :)
• Honestly this is kinda lame like were never gonna do this in the real world Right?
• My mom randomly makes me Do it
• why do we do these
• FOR REAL why do we do these are we gonna do this in real life like
• at how did you get 1/2 dont u do 6 divided by 3 is 2? im a little confused?
• 3/6 is the same as 1/2 because 3 times 2 is 6
• 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?
• Yes that is correct! You’ve shown that you understand the graphical interpretation of the constant of proportionality.
• 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?
• 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.
• 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
• If you are starting with: 4y=8x, you need to divide both sides by 4 to find the constant of proportionality. You get:
y = 2x
The constant of proportionality = 2
The value of Y will always be 2 times the value of X.

Hope this helps.