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Homework Help: What is the locus (unsure if this is the right word) defined by these equations?

  1. Sep 9, 2011 #1
    1. The problem statement, all variables and given/known data

    Using the Ax+By+Cz+D= 0 equation for planes, where D = -(Axo, Byo, Czo)

    Find the locus given by the equation if A and B = 0

    note: I'm not 100% sure if locus is the right word, english isn't my primary language.

    2. Relevant equations

    Given above.

    3. The attempt at a solution

    Well, if A and B = 0, the equations can be written as Cz+D = 0.

    Which means z = -D/C = (0x + 0y Czo)/C = zo

    Since we're in a 3 dimensions space, the way I see it both x and y are unrestricted and can take any values. Does that mean the locus defines a plane parallel to the X and Y axis, at a Z value of Zo?
  2. jcsd
  3. Sep 9, 2011 #2
    One more thing. The book gives an example, stating that if A=0 then the equation defines a plane parallel to the X axis. Shouldn't it say perpendicular, instead of parallel?
  4. Sep 9, 2011 #3


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    Your answer, that's it's a plane parallel to the X and Y axes, is correct. But why are you writing D = -(Axo, Byo, Czo)? That looks like a vector to me. That doesn't make sense, a form like Ax+By+Cz+D=0. D should be a scalar, i.e. a simple number.
    Last edited: Sep 9, 2011
  5. Sep 9, 2011 #4


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    The plane z=1 is parallel to x axis, isn't it? That has A=0.
  6. Sep 9, 2011 #5
    Yes sorry, I meant D = - (Axo + Byo + Czo),

    A,B and C being the components of the normal vector N=(A,B,C)
    and xo, yo, zo being the coordinates of a point on the plane Po:(xo,yo,zo).
  7. Sep 9, 2011 #6
    Yeah I noticed my mistake after posting that. I somehow misunderstood A=0 for x=0. Thanks for the help!
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