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Homework Help: Linear Law: I'm confused.

  1. Aug 24, 2010 #1
    Okay, I was doing some questions concerning linear law when I came across a question that asked me to convert a cubic equation into a 'linear equation' so that I can draw it on a graph.

    Cubic Equation:

    p(x+y-q) = qx3

    Then, I convert the above equation into:

    y/x2 = (q/p)x

    However, when I tried plotting the graph. Letting y/x2 to be the y-axis and x to be the x-axis, I got a curve, instead of a line!

    What's wrong?
  2. jcsd
  3. Aug 24, 2010 #2
    But then your y-axis is dependent on your x-axis, that's VERY odd.

    The procedure you have to go through is probably more like the following:

    Say y = e^x and we want to represent is linearly, what do we do? Well we know that to get x downstairs, we can use the ln function: ln(y) = x

    Now we draw a regular y' = x and on the y'-axis we write y' = ln(y)

    So it basically comes down to: changing your variables! I've just done an example where I replaced y, but I think I'd replace x in your example, so that you have to write on the x'-axis: x' = f(x); what is f?
  4. Aug 24, 2010 #3

    The answer given to me is:

    x+y = (q/p)x3 + q

    Letting x+y to be the y-axis and x3 to be the x-axis, I indeed got a straight line graph...
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