1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook