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Math Help - zeros of polynomials

  1. #1
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    Thumbs up zeros of polynomials

    Solve:
    x^4-10x^3+42x^2-82x+65=0 the product of two of the roots of this equation is 13. The answers are complex numbers.
    Thanks for helping me
    Last edited by Anand Parikh; May 18th 2008 at 04:11 AM.
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  2. #2
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    Quote Originally Posted by Anand Parikh View Post
    Solve:
    x^4-10x^3+42x^2-82x+65=0 the product of two of the roots of this equation is 13. The answers are complex numbers.
    Thanks for helping me
    Vieta's Root formulas should help here ..... Start with the product of all the roots being equal to 65 .......
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  3. #3
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    Quote Originally Posted by Anand Parikh View Post
    Solve:
    x^4-10x^3+42x^2-82x+65=0 the product of two of the roots of this equation is 13. The answers are complex numbers.
    Thanks for helping me
    Because this is a quartic with real coefficients this factorises into a pair of real quadratics:

    x^4-10x^3+42x^2-82x+65=(x^2+a x +b)(x^2+ c x +d)

    where a, b, c, d are all real. Also as the product of a pair of roots is 13, this suggests we try b=13 and d=5. Doing this we find:

    x^4-10x^3+42x^2-82x+65=(x^2-6x+13)(x^2-4x+5)

    and from there it is easy.

    RonL
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  4. #4
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    Hello, Anand Parikh!

    Solve: . f(x) \:=\:x^4-10x^3+42x^2-82x+65\:=\:0
    The product of two of the roots of this equation is 13.
    The roots are complex numbers.
    I approached it like CaptainBlack and got very lucky . . .

    Since the product of two roots is 13,
    . . I guessed that the roots were: . 2 \pm 3i\,\text{ or }\,3\pm2i


    If the roots are: . 2 \pm3i
    . . then: . \left(x - [2 + 3i]\right)\,\text{ and }\,\left(x-[2-3i]\right) .are factors of f(x).
    Hence: . (x-2-3i)(x-2+3i) \:=\:x^2-4x+13 .is a factor of f(x).

    . . But x^2-4x+13 does not divide f(x).


    If the roots are: . 3 \pm2i
    . . then: . \left(x - [3+2i]\right),\text{ and }\, \left(x - [3-2i]\right) .are factors of f(x).
    Hence: . (x-3-2i)(x-3+2i) \:=\:x^2 -6x + 13 .is a factor of f(x).

    . . And we have: . f(x)\;=\;(x^2-6x+13)(x^2-4x+5) \quad\hdots\quad \text{ta-}DAA!

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