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: Which step am I doing wrong?

  1. Sep 28, 2012 #1
    1. The problem statement, all variables and given/known data

    limi as x-> positive infinity x+r((x^2)+2x)

    3. The attempt at a solution

    multiply by conjugate x-r((x^2)+2x)

    I get (x^2)-(x^2)+2x/x-r((x^2)+2x)

    Which becomes 2x/x-r((x^2)+2x)

    Which I end up with 2/1-r(1-(2/x))

    But I go wrong somewhere because I end up with 2/0 and the answer is positive infinity. I keep getting undefined and I can't see where I'm going wrong. Could someone point it out?
  2. jcsd
  3. Sep 28, 2012 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    What is r? Is it a number, a function, or what? If it is a number > 0 you should have no trouble saying what is ## lim_{x \rightarrow \infty} x + r(x^2 + x).## If r is a number < 0 it is almost as easy, but you need to be careful. And, of course, if r = 0 it is easier still.

    I have no idea why you would want to take conjugates, and anyway, I stopped reading your work because you have not used brackets I cannot tell whether you mean
    [tex] x^2 - x^2 + \frac{2x}{x} - r(x^2 + x),[/tex] (which IS the meaning of what you wrote) or whether you mean
    [tex] x^2 - x^2 - \frac{2x}{x - r(x^2 + 2x)}[/tex]
    [tex] x^2 - \frac{x^2 + 2x}{x - r(x^2 + 2x)},[/tex]
    or several other possibilities.

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