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The roots of x^8 - 5x^6 + 7x^4 - 5x^2 +6=0

  1. Sep 4, 2015 #1
    [Thread moved to homework forum by a Mentor]
    the exercise was to find the roots of x^8 - 5x^6 + 7x^4 - 5x^2 +6=0

    I substituded x^2 with y
    : y^4 - 5y^3 + 7y^2 - 5y +6=0

    I factored this by doing the rational roots test and trying those possible roots with the method of horner

    and got (y-2) (y-3)(y^2+1)=0

    with y=x^2 ---> (x^2-2) (x^2-3)(x^4+1)=0

    i found the roots ±√2 , ±√3. which is correct and the answer on the back says these are the 2 anwsers. but (x^4+1) remains --> what are the 2 complex roots? how do I change 4^√-1 in a notation with i?
     
    Last edited by a moderator: Sep 4, 2015
  2. jcsd
  3. Sep 4, 2015 #2

    HallsofIvy

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    [itex]x^4+ 1= 0[/itex] is, of course, the same as [itex]x^4= -1[/itex] so the solutions are the four fourth roots of -1. You can find them using DeMoivres theorem: The nth roots of [itex]r(cos(\theta)+ i sin(\theta)[/itex] are [itex]r^{1/n}(cos(\frac{1}{n}(\theta+ 2k\pi)+ isin(\theta+ 2k\pi))[/itex] where k runs from 0 to n- 1.

    Here, r= 1 and [itex]\theta= \pi[/itex].
     
  4. Sep 4, 2015 #3

    BvU

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    Ah, and welcome to PF, dear mathnovice :smile: ! Good attitude to ask a little more !


    It's a fourth order, so you in fact expect four solutions !

    Given that ##i^2 = -1## you are left with solving ##x^2 = i ## or ##x^2 = -i ##

    Now it becomes a little weirder: one solution for the first one is ##x = {1\over \sqrt 2}(1+i) ## !

    Check by writing out: ##x^2 = {1\over 2}(1+i)(1+i) = {1\over 2}(1^2+2i+i^2) = {1\over 2}(1+2i-1) = i##

    I could go on, but maybe you would like to find the other three solutions by yourself ?

    Then draw the solutions in a Cartesian coordinate system where instead of x and y you have the real part of x (so ##{1\over \sqrt 2}## in my example ) horizontally and the imaginary part of x vertically. Welcome to the world of imaginary numbers !
     
  5. Sep 5, 2015 #4

    Ray Vickson

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    To amplify a bit on what others have told you: we can immediately find two 4th roots of ##-1## by noting that
    [tex] -1 = e^{\pm i \pi} \Longrightarrow (-1)^{1/4} = e^{\pm i \pi/4} = \frac{1 \pm i}{\sqrt{2} }[/tex]
    However, if ##r## is a fourth root of ##-1## then so is ##r^3##; do you see why? This gives us two other fourth roots
    [tex] e^{\pm 3 i \pi/4} = \frac{? \pm i \,?}{?} [/tex]
    You can fill in the remaining details.
     
    Last edited: Sep 8, 2015
  6. Sep 5, 2015 #5

    BvU

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    With this wording, I expect the poster to be helped with an explanation at an introductory level. Dear novice, did the replies make you any wiser ?
     
  7. Sep 7, 2015 #6
    Everybody thanks for the replies. I am now improving my knowledge about complex numbers first and will then tackle this problem. But it did motivate me to discover the whole subjet of complex numbers, as I just find out complex numbers are also used in engineering applications and that's my goal for improving my mathknowledge ( I'm going to study engineering at university next year )
     
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