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Hyperbola or ellipse problem

  1. Jan 20, 2016 #1
    1. The problem statement, all variables and given/known data

    The following question is posed within a section of my A level maths book titled "The Hyperbola"

    A set of points is such that each point is three times as far from the y axis as it is from the point (4,0). Find the equation of the locus of P and sketch the locus

    2. Relevant equations


    3. The attempt at a solution

    If P is a point (x,y) on the locus, and N is the intersection on the y axis of the line through P, parallel to the x axis, and S is the point (4,0) then
    [itex]
    PN = 3PS\\

    PN^2 = 9PS^2\\
    x^2 = 9((x-4)^2+y^2)\\
    \frac{8}{9}x^2+y^2-8x+16=0\\
    [/itex]
    Which I believe is an elipse, but my book indicates that it is a hyperbola with it's answer of
    [itex]
    8x^2-y^2+8x-16=0
    [/itex]

    Is my book wrong?
     
  2. jcsd
  3. Jan 20, 2016 #2

    Ray Vickson

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    Yes, YOU are right. You can even plot the curve in some package such as Maple to see what is happening.

    You can even argue intuitively that the curve must be bounded in the plane, because if you could take ##x \to \infty ## very large (and ##y## moderate) on the curve you would have have (approximately) ##x \approx 3 (x-4)##, so ##x \approx 6##, contradicting the condition that ##x \to \infty## is very large.
     
    Last edited: Jan 20, 2016
  4. Jan 21, 2016 #3
    Thanks, it's encouraging to know that writers of maths books fall prey to the same kind of mistakes that I do, if a little less frequently.
     
  5. Jan 21, 2016 #4

    Samy_A

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    Can happen. The hyperbola they give as result is the solution of the exercise "A set of points is such that each point is three times as far from the y axis point (4,0) as it is from the point (4,0) y axis."
     
  6. Jan 21, 2016 #5

    Ray Vickson

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    It would have been a hyperbola if it had said "... is 3 times as far from the point (4,0) as from the y axis".
     
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