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Precalculus: Writing polynomial functions (help!)

  1. Sep 26, 2006 #1
    Precalculus: Writing polynomial functions (help!) :(

    Hey guys and gals, i'd appreciate if you could help me (well) with this question.

    Write a polynomial function at minimum degree in standard form with real coefficients whose zeros include -1 and 1+4i

    :O its hard for me, im not a math person, and my precalculus teacher doesn't teach, he just talks.

    Thanks!

    btw i have a test tomorrow, so if you could tell me the answer and how you got it, that'd be awesome, because then i can study it and learn, thanks!
     
  2. jcsd
  3. Sep 26, 2006 #2

    quasar987

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    First, can you answer the question "what is the minimum degree that a polynomial can have if it has 2 roots" ?
     
  4. Sep 26, 2006 #3

    No i cannot. What is it?
     
  5. Sep 26, 2006 #4

    quasar987

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    Well it's 2. It's can't be one, because a polynomial of degree 1 is of the form

    x+b=0 ==> x=-b is the only root.

    It can be two though because a polynomial of degree 2 with roots a,b can be writen as (x-a)(x-b)=0. But if you set a=-11, b=1+4i, and expand the multiplication of the parenthesis, you'll see that the coefficients are not real. So maybe the minimum degree is 3, in which case the polynomial will be of the form (x+1)(x-(1+4i))(x-z)=0 where z is a 3rd root. Can you choose z such that all the coefficients are real? I've done it and I can't seem to find one, so perhaps it is of degree 4.
     
  6. Sep 26, 2006 #5

    quasar987

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    I'm sorry, I'm not much help :p
     
  7. Sep 26, 2006 #6
    Yeah i know, :P

    Do you think you can simply tell me step-by-step how to do the problem? I dont really understand much of the pre-cal jargon, at all. Im a simple person, :( unfortunately, so if you can, tell me how to do it step-by-step, plz.

    thanx.
     
  8. Sep 26, 2006 #7

    quasar987

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    I don't know how!
     
  9. Sep 26, 2006 #8

    quasar987

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    I told you about my idea, but it seems impracticable. There must be a simpler way.
     
  10. Sep 26, 2006 #9
    its alright, i'll ask someone else.

    Thanks for trying though, :D
     
  11. Sep 27, 2006 #10

    HallsofIvy

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    You know how to solve polynomial equations, don't you? The simplest way is to factor them.

    If a polynomial has a as a zero, then it has x-a as a factor.

    Also, if a polynomial, with real coefficients, has a+ bi as a zero, it also has a- bi as a factor.

    If a polynomial, with real coefficients, has 1+ 4i as a zero then it must also have 1- 4i as a zero. So your polynomial has x+ 1, x- 1- 4i, and x- 1+ 4i as factors. Multiply them.
     
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