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Homework Help: Eliminating the parameter

  1. Jun 26, 2011 #1
    I have the parametric function x(t) = (1-t^2)/(1+t^2), y(t) = 2t/(1+t^2) and need to eliminate the parameter and find a Cartesian equation.

    I've tried to substitute t = tan u, then x(t) = cos(2u) and y(t) = tan(2u). From that I get y = sin(2x)/x. However, when I entered the original parametric function into a grapher, I get an entirely different graph. Where did I go wrong?
  2. jcsd
  3. Jun 26, 2011 #2


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    Homework Helper

    I'm not getting that for the y(t) equation. I think it's because you got your identity confused.
    [tex]\frac{2\tan u}{1 - \tan^2 u} = \tan 2u[/tex]
    (minus in the denominator)

    But here we have:
    [tex]y(t) = \frac{2\tan u}{1 + \tan^2 u} = \frac{2\tan u}{\sec^2 u} = ...[/tex]
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