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May 27, 2022 at 13:00 comment added Benjamin Wang @whuber cool visualisation
Jan 8, 2015 at 3:31 comment added whuber Re the visualization: Imagine the intersection of a spherical bowl with a bowl that is so long and shallow (but with steep sides) that it looks almost like a folded piece of paper. That is what things look like when $b=2/a$ and $c=1$. The steep sides of the paper rise above the bowl's sides, but the valley along the center of the paper falls below the bowl's sides. Thus, the region where the paper is higher consists of two separate areas delimited by the arcs of the hyperbola.
Mar 17, 2013 at 22:49 history edited Dilip Sarwate CC BY-SA 3.0
corrected speculations in last paragraph
Mar 17, 2013 at 16:06 comment added whuber Good beginning! I fixed a subtraction error in the final formula. As a result, you might want to revisit your speculations in the last paragraph. To get started, note that the equation is linear if and only if all the quadratic coefficients are zero, which is equivalent to $a=1$, $b=1$, and $c=0$. (There is another special case where the equation represents a pair of lines.) Whether the locus is elliptical or hyperbolic depends on the sign of the determinant $(a-1)(b-1)-c^2$; both signs are possible.
Mar 17, 2013 at 16:02 history edited whuber CC BY-SA 3.0
added 13 characters in body
Oct 18, 2012 at 12:44 history edited Dilip Sarwate CC BY-SA 3.0
cleaned up the peroration
Oct 17, 2012 at 21:31 history answered Dilip Sarwate CC BY-SA 3.0