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Math Help - Need help with roots of a equation

  1. #1
    L44
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    Need help with roots of a equation

    Question :

    Give that α & β are roots of the equation 2x2 + 5x - 7 = 0

    Find without solving the equation the value of (α -β)2

    Need to calculate the sum of the root and the product of the roots
    Last edited by L44; November 18th 2012 at 11:35 AM.
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  2. #2
    MHF Contributor MarkFL's Avatar
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    Re: Need help with roots of a equation

    Vieta's formulas give us for the quadratic ax^2+bx+c having roots \alpha,\,\beta:

    \alpha+\beta=-\frac{b}{a}

    \alpha\beta=\frac{c}{a}
    Last edited by MarkFL; November 18th 2012 at 12:07 PM. Reason: Thanks HallsOfIvy!
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  3. #3
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    Re: Need help with roots of a equation

    MarkFL meant to write ax^2+ bx+ c
    Thanks from MarkFL
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  4. #4
    L44
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    Re: Need help with roots of a equation

    I know that part i need to find the quadratic equation with the roots (α -β)2
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  5. #5
    MHF Contributor MarkFL's Avatar
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    Re: Need help with roots of a equation

    No, you need to find the value of (\alpha-\beta)^2=(\alpha+\beta)^2-4\alpha\beta.
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  6. #6
    L44
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    Re: Need help with roots of a equation

    Yeah I have that part as well the part I am not sure about is the sum of the root
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  7. #7
    MHF Contributor MarkFL's Avatar
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    Re: Need help with roots of a equation

    Since you are new here, I am going to give you a few tips:

    1.) Don't double post. You have posted this topic in 2 different forums. This can lead to duplication of effort on the part of our contributors. You should choose the forum most appropriate for your question, and post it once there.

    2.) Let us know what you have already done, and exactly where you are stuck. This avoids us telling you what you already know, and expedites the process.

    So, you need the sum of the roots...you say you know Vieta's formulas, but these formulas give you just what you need, i.e., the sum and product of the roots.
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  8. #8
    L44
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    Re: Need help with roots of a equation

    well the double posting was not intentional I thought I placed the original post in the wrong thread where no one would see it.

    I found α + β = -b/a = -5/2 while αβ = c/a = -7/2

    the product of the root --> ((α -β)^2 = (α -β)(α -β) = (α+β)^2 - 4αβ= when you substitute the values where appropriate the final answer is 20.25
    the sum of the root is (α -β) + (α -β) = 2α - 2β and this is the part that I am stuck
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  9. #9
    MHF Contributor MarkFL's Avatar
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    Re: Need help with roots of a equation

    In the future if you find you have posted in the wrong forum, you may use the report post function at the lower left of each post to report the post to the moderators and ask them to move the topic to the appropriate forum.

    You have found that the requested expression is 20.25, which is correct. I don't understand why you are doing the additional step.

    What is the problem in its entirety, exactly as given?
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