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Homework Help: Ellipse equation help problem

  1. Dec 7, 2008 #1
    1. The problem statement, all variables and given/known data

    an ellipse has the vertices (+&-4, 0)and the point (1,2) lies on the ellipse. find the standard form of the equation of the ellipse .

    2. Relevant equations

    (x-h)^2/a^2 + (y-k)/ b^2 = 1

    3. The attempt at a solution
    (x-0)^2/(4)^2 + (y-0)/b^2 = 1
    (1-0)^2/16 + (2-0)^2/b^2 = 1
    16b^2 ( 1/16+4/b^2=1)
    **b^2 +64/16 = 16b^2
    ** b^2/16 + 4 = b^2
    **b=b/4 + 2
    note: *** is where i think i made my mistakes.
    so will anyone tell me, what i did wrong and help me with how to finish this problem, thanks .
  2. jcsd
  3. Dec 7, 2008 #2


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    Homework Helper

    Substitute the vertex first, then you'll find what a^2 works out to be.
    After that, then sub in the point (1,2)
  4. Dec 7, 2008 #3
    Here, your brackets are in the wrong place and if you expand you'll see that it doesn't match the equation above it, which means your factoring was off.

    [tex]\frac{1}{16} + \frac{4}{b^2} = 1[/tex]

    To isolate for b, move all the constants to one side and then cross multiply.
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