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Sketching x/[(1+3x)^2-1]

  1. Sep 30, 2009 #1

    I'm wondering if it's even possible to imagine what a graph of this fxn would look like, or do you definitely need a graphing calc?

    When I plug it into a TI83, it ends up looking pretty linear and apparently doesn't exist when x < 0


    PS. sorry for how messy the eq'n looks, it's difficult to type. ... (1+3x) should be under a sqrt. sign.
  2. jcsd
  3. Sep 30, 2009 #2
    Just by looking at it, you see that you have x in the numerator and x1/2 in the denominator. For large x, the function will roughly look like x1/2, but you can find a better approximation. After multiplying the top and bottom of the fraction by the conjugate of the denominator and doing some work on it, you get

    [tex]\frac{\sqrt{3x + 1} + 1}{3} = \frac{\sqrt{3x(1 + \frac{1}{3x})} + 1}{3} = \frac{\sqrt{3x}\sqrt{1 + \frac{1}{3x}}}{3} ~+~ \frac{1}{3} = \frac{\sqrt{3}}{3}\sqrt{x} \sqrt{1 + \frac{1}{3x}} ~+~ \frac{1}{3}[/tex]

    As x→∞, the larger radicand goes to 1 and has less and less effect on √x, so the function is nearly like [tex]\frac{\sqrt{3}}{3}\sqrt{x} ~+~ \frac{1}{3}[/tex]
  4. Oct 6, 2009 #3
    thanks ^_^
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