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Math Help - [SOLVED] Proof of algebra for ellipse

  1. #1
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    Apr 2010
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    [SOLVED] Proof of algebra for ellipse

    Hello.
    This is the same problem for me

    http://www.mathhelpforum.com/math-he...f-ellipse.html

    I find the same values of:

    a=5
    b=sqrt(15)
    Foci: + and - (sqrt(10), 0)
    Directrices: + and - 5*sqrt(10)/2
    Eccentricity: sqrt(10)/5
    Center: (0,0)
    Vertices: (5,0) (-5,0) (0, sqrt(15)), (0, -sqrt(15))

    I only worry about the PF=ePd part of the question.

    Does not this mean P * F = e * P * d?

    When I am putting all of the numbers in the equation then the two sides are not being equal. My answer is:

    (5 * sqrt(10) = sqrt(10)/5 * 5 * 25/sqrt(10)

    so that (5 * sqrt(10) = 5 * sqrt(10)/2

    When I look at the above link, they are using different equations (where do they find this?) and the answers result differently. They use

    PF = a-ae = -5+\sqrt{10}
    and ePd = e(a-ae)/e = \sqrt{10}/5\times(-5+\sqrt{10}/\sqrt{10}/5
    = -5+\sqrt{10}

    This is not like I have. How must I find "PF =a-ae" and "ePd=e(a-ae)/e"? It is not in my book and I am not finding it on the internet.

    My sides don't balance. The sides of him balance!

    Hope someone is not busy to help me!
    Brigitte
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  2. #2
    Newbie
    Joined
    Apr 2010
    Posts
    17
    Now I have solved this.

    Thanks.
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