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Homework Help: Geometry Coordinate

  1. Aug 22, 2007 #1
    1. The problem statement, all variables and given/known data
    Given that Z is a complex number with condition |Z-1|+|Z+1|=7

    Illustrate Z on Argand Diagram and write out the equation of Locuz Z

    I attempted to figured out the equation of locus Z,
    [tex]\sqrt{}[(x-1)^2+y^2][/tex] + [tex]\sqrt{}[(x+1)^2 + y^2][/tex] = 7
    [tex]\sqr{}x^2 + 1 - 2x + y^2 + x^2 + 1 + 2x + y^2 = 49[/tex]
    [tex]\sqr{}2x^2 + 2y^2 = 47[/tex]

    it's not necessary the correct answer though...
    however, I can't figure how to illustrate the diagram! help!
  2. jcsd
  3. Aug 22, 2007 #2
    Assuming that your calculations are correct, that gives a circle of radius [tex]\sqrt{47/2}[/tex]. However, I don't think it is... Check your algebra carefully -- squaring both sides doesn't mean get rid of square roots!

    Another way to think about it is that the original equation says that the distance from a point on the locus to the points +1 and -1 add up to 7. This is the condition for an ellipse with its foci at -1 and 1! And an ellipse is only a circle if the foci coincide.
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