algebra

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Hi I'm having trouble with this question I don't understand it at all. This is the question:
Suppose y varies directly with x, and y = 15 and x = 5. Write a direct variation equation that relates x and y. What is the value of y when x = 9?

If somebody could help me & let me know what a direct variation equation is I'll be very grateful to you! I don't know what this is, I missed the last 2 days of school & that must have been when the teacher went over this. My friend tried to help me but he doesn't know what direct variation means either. Thank you if you know anything about this :)))

  • algebra -

    It's homework due tomorrow & I have practice in an hour so if you can help me before then I'll be so grateful!

  • algebra -

    Okay I found something on this, so you do 15=5k then divide both sides by 5 to solve for k so you get k=3 then do y=3x but put what they gave, so x=9 so then y=3(9) so is the answer y=27??

  • algebra -

    x = yk

    5 = 15 * 1/3

    9 = y * 1/3

    Although I use a slightly different process, our answers are the same.

  • algebra -

    Whenever you have a direct variation of something like
    (.....y....) varies directly with (.....x.....) we can say
    y = kx

    Usually you will be given one case where that is true, in your case x = 5 and y = 15.
    So sub that in ...
    15 = 5k
    k = 3

    so now we know the variation equation is y = 3x
    The question was : What is the value of y when x = 9?
    so let's put in x = 9
    y = 3(9) = 27

    If the variation is inversely proportional,
    we say y = k(1/x) and proceed as before.

  • algebra -

    Yay, so I was right? Thank you so much

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