text: How can you tell which axis the graph is oriented on? Obviously this is very easy for a hyperbolic paraboloid, but what about for the equation x^2+y^2-z^2=0?
  
  2. Originally Posted by AK713
  How can you tell which axis the graph is oriented on? Obviously this is very easy for a hyperbolic paraboloid, but what about for the equation x^2+y^2-z^2=0?
  x^2 +y^2 -z^2 = 0 --------(i)
  Rearranging that,
  x^2 +y^2 = z^2 ----------(ii)
  The LHS of (ii) is a circle.
  The RHS is a square of a z-coordinate. It could be the square of the radius of the LHS at the z-coordinate.
  
  When z=0, the circle at the LHS is zero too, or no circle. It is (0,0,0), or the origin.
  
  When z = 1, or z = -1, the circle at the LHS has a radius of 1.
  When z = 2, or z = -2, the circle at the LHS has a radius of 2.
  When z = 3, or z = -3, the circle at the LHS has a radius of 3.
  Etc...
  
  The figure then is that of a cone whose apex is at the (0,0,0) and opening upwards, and that of its mirror-shape that opens downward.
  That is if the z-axis is the usual up-down or vertical axis in the x,y,z axes setup.