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Homework Help: What is this? ;)

  1. Nov 28, 2008 #1
    z = x2-y2
    How do I draw it?
  2. jcsd
  3. Nov 28, 2008 #2
    How would you draw any graph? Answer: By making a table of values and plotting them.
  4. Nov 28, 2008 #3
    yes I say that z = const and look further
  5. Nov 28, 2008 #4


    Staff: Mentor


    But z is not a constant, according to the equation in your first post.

    A more sophisticated approach than that of the other responder is to look at a variety of cross-sections. The graph of z = x^2 - y^2 is a three-dimensional surface. The intersection of the surface with the x-z plane (where y = 0) is a parabola, and so is the intersection of the surface with the y-z plane (where x = 0).

    Look at the intersections with the planes y = +/-1, +/-2 and so on, as well as the planes x = +/-1, +/-2. These might help you visualize what the surface looks like.
  6. Nov 28, 2008 #5
    I wouldn't. I would set y = 0. Draw that like you would in high school z = x^2, looks like a parabola in the x going up /wrt z. Do the same for y = 0, except you get a parabola going down in the yz plane. I would say you can stop there. Draw one of the parabolas in xyz system. Suppose you choose z = -y^2. Then you get a parabola that is going down around the z axis. Now attach one of your z=x^2 parabolas to some points that lie on the z = -y^2 parabola. You can generally get a frame for a 3d graph like this. If you are really careful, the graph might look like a horses saddle.
  7. Nov 29, 2008 #6
    yeah, that's what i was thinking - it might be a horse saddle !!!
    But I wasn't sure about how to draw it carefully. Thank you all.
    Last edited: Nov 29, 2008
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