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Simple equation causing a lot of head ache!

  1. Jan 23, 2009 #1
    Hi there!

    I was trying to solve an equation but got very perplexed by the fact that a certain number x=0 is both a solution and no solution:

    Here´s the equation:

    [tex](x-a)[(x-a)^2+y^2+z^2]^{-3/2}+(x+a)[(x+a)^2+y^2+z^2]^{-3/2}=0[/tex]

    assume y=z=0:

    Now, the equation becomes:

    [tex](x-a)[(x-a)^2]^{-3/2}+(x+a)[(x+a)^2]^{-3/2}=0[/tex]

    I guessed a solution at x=0

    check: [tex]-a[(-a)^2]^{-3/2}+a[a^2]^{-3/2}=0[/tex], ok I assume it´s true

    Now let´s use the exponent rule: (a^x)^y=a^(xy)

    Then the equation becomes:

    (*) [tex](x-a)(x-a)^{-3}+(x+a)(x+a)^{-3}=0[/tex], or

    [tex](x-a)^{-2}+(x+a)^{-2}=0[/tex]

    Ok, plug once again x=0 and there coems the surprise:

    [tex](-a)^{-2}+(+a)^{-2}=1/a^2+1/a^2[/tex] is not equal to 0!!!

    What is more, if you substitute x=0 before you contract the factors in (*) ypu come up with: [tex](-a)^{-4}+(+a)^{-2}=1/a^4+1/a^2[/tex] again no 0, but also different from the result above!


    I suppose I´m doing it somehow wrong woth the powers and exponents but I can´t figure it out!

    If you see my mistake, please tell me!

    Thanks in advance!
     
  2. jcsd
  3. Jan 23, 2009 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi Marin! :smile:
    Sorry … doesn't always work, especially if y is a fraction and a is negative :wink:
     
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