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help:find roots

  • 31-01-2010 10:56pm
    #1
    Chewbacca.
    Closed Accounts Posts: 190 ✭✭


    please someone tell me the easiest way to find roots of
    8x^4 - 7x^2 + 1 =0
    ?


Comments

  • #2
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    Use a substitution to transform it to a quadratic. Any ideas?


  • #3
    Chewbacca.
    Closed Accounts Posts: 190 ✭✭Chewbacca.


    yes I used y as x^2
    but I can only get 2 roots x=1 or x=0.35?
    instead of 4roots???


  • #4
    Pherekydes
    Registered Users, Registered Users 2 Posts: 16,201 ✭✭✭✭Pherekydes


    Chewbacca. wrote: »
    yes I used y as x^2
    but I can only get 2 roots x=1 or x=0.35?
    instead of 4roots???

    You've solved for y; now sub back in to get the 4 roots of x.


  • #5
    Chewbacca.
    Closed Accounts Posts: 190 ✭✭Chewbacca.


    Pherekydes wrote: »
    You've solved for y; now sub back in to get the 4 roots of x.
    would you be kind enough to show me how to do this?


  • #6
    Chewbacca.
    Closed Accounts Posts: 190 ✭✭Chewbacca.


    Oh really golly gosh how helpful of you all,
    what an excellent forum to frequent.


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  • #7
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    Chewbacca. wrote: »
    Oh really golly gosh how helpful of you all,
    what an excellent forum to frequent.

    Excuse me, firstly, read the charter, no one is here to do anything for you, just to point you in the right direction and help with understanding. Secondly, it's a chat forum, when a reply is not instantaneous that does not mean someone is ignoring you, it just means they're currently not browsing the forum. The poster could be in bed, could be having some food, whatever. I've a mind to ban you for just being an ass but I won't this time. Cop on to yourself.


  • #8
    Thomas_S_Hunterson
    Closed Accounts Posts: 6,151 ✭✭✭Thomas_S_Hunterson


    Chewbacca. wrote: »
    Oh really golly gosh how helpful of you all,
    what an excellent forum to frequent.

    Link


  • #9
    jaysond
    Closed Accounts Posts: 10 ✭jaysond


    Good explanation of how to solve a biquadratic function.

    http://en.wikipedia.org/wiki/Quartic_equation

    Like the URL Sean_K!!


  • #10
    Fremen
    Registered Users, Registered Users 2 Posts: 2,481 ✭✭✭Fremen


    Solve y^2 = a for some sensible value of a.
    We don't do your work for you here.

    I bet you're a joy to teach.


  • #11
    Chewbacca.
    Closed Accounts Posts: 190 ✭✭Chewbacca.


    I bet you all have really ugly faces????????


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  • #12
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    Chewbacca. wrote: »
    I bet you all have really ugly faces????????

    And here was me hoping you'd not be a child. Banned!


  • #13
    Pherekydes
    Registered Users, Registered Users 2 Posts: 16,201 ✭✭✭✭Pherekydes


    Sean_K wrote: »
    Link

    Beaut! :D

    Did anyone read the parallel thread in the LC forum? This is one impatient little puppy.

    Edited to add: Yes, I did go to bed shortly after my previous post at 23.40. Well, I need my beauty sleep. :o


  • #14
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    How dare you go to bed and leave a question unanswered!


  • #15
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    Also had a look in that thread and some kid posted the way to do this is to guess the roots cause they never go past plus/minus 3 :eek: What do they teach these kids in school?


  • #16
    Fremen
    Registered Users, Registered Users 2 Posts: 2,481 ✭✭✭Fremen


    Sean_K wrote: »
    Link

    The first link on that google search is this page.
    *Head asplode*


  • #17
    RoundTower
    Registered Users, Registered Users 2 Posts: 5,083 ✭✭✭RoundTower


    LeixlipRed wrote: »
    Also had a look in that thread and some kid posted the way to do this is to guess the roots cause they never go past plus/minus 3 :eek: What do they teach these kids in school?

    they do teach that, at least for cubics, and it is certainly the first thing you should try. I don't think you can get a quartic on the LC without the really obvious substitution.


  • #18
    LeixlipRed
    Closed Accounts Posts: 6,081 ✭✭✭LeixlipRed


    Ah of course, for a monic polynomial with nice apparent factors yeh but this poster was suggesting it's the only way to do it. That's what hurts my head.


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