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Math Help - another variation

  1. #1
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    another variation

    find the required value by setting up the general equation and then evaluating.

    find y when x=10 if y varies directly as x and y=20 when x=8


    thanks!
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  2. #2
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    Quote Originally Posted by mamajen View Post
    find the required value by setting up the general equation and then evaluating.

    find y when x=10 if y varies directly as x and y=20 when x=8


    thanks!
    When y varies directly as x, we have the equation y = kx. The first thing to do is solve for k. Substituting in y = 20 and x = 8 we have 20 = 8k, or k = \frac{20}{8} = \frac{5}{2}.

    Given that y = \frac{5}{2}x, can you determine y when x = 10?
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  3. #3
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    yes i can get it from there. thanks. I was not thinking to find k first!
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  4. #4
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    Icemanfan, hope you don't mind me just adding something.

    mamajen, In the other question you asked, I said what "proportional" mean and told you about putting the k there and then working out what k was. Well, "varies directly as" just means "is proportional to", so the two questions are quite similar.
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  5. #5
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    thanks iceman. i remembered that part but didn't know where to put all the equal to #'s. then when I saw he used the 2nd y= to find k I knew where I was going
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