Stuck on Algebra 1

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This is kind of like the first question I asked, and I'm still having a little trouble on it.

The Ordered pairs in each exercise are for the same direct variation.
Find the missing value.

(-2,8) and (x,12)

I'm not sure how to work out the problem.

  • Stuck on Algebra 1 -

    I'm not sure but if i cross multiply the two coordinates, i think the answer is...

    x=-3?

  • Stuck on Algebra 1 -

    Cross-multiplying seems to be the process:
    http://www.jiskha.com/display.cgi?id=1199568727

    So ...

    -2/8 = x/12
    8x = -24
    x = -3

    Right?

  • Stuck on Algebra 1 -

    yes, x=-3

  • Stuck on Algebra 1 -

    Although I am sure I will be called on the carpet for saying why, I will try anyway.
    If it is a direct variation it is a straight line of form:

    y = k x
    put in your first point
    8 = -2 x
    so k = -4
    NOW any other point on this line satisfies
    y = -4 x
    so put in 12 for y
    12 = -4 x
    x = -3 as you know from "cross multiplying"

  • Stuck on Algebra 1 -

    yes, that's the way I didn't understand how to do before.

  • Stuck on Algebra 1 -

    By the way, these problems are a special case of direct variation, linear.
    The relation could be quadratic, cubic quartic ....
    but is always of form y = k x^n
    Here so far we are assuming that n = 1

  • Stuck on Algebra 1 -

    Do you get it now?

  • Stuck on Algebra 1 -

    Um, i did until you mention the linear part.

  • Stuck on Algebra 1 -

    Fixing typo and expanding

    y = k x
    put in your first point
    8 = -2 k
    solve that for k
    so k = -4
    NOW any other point on this line satisfies
    y = -4 x
    so put in 12 for y
    12 = -4 x
    x = -3 as you know from "cross multiplying"

  • Stuck on Algebra 1 -

    I do not think you will get direct variation that is quadratic etc in this course or they would have mentioned it. For now assume direct variation means y = k x

  • Stuck on Algebra 1 -

    Ok , thank you for your help.

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