What does this symbol mean

Question

What does the symbol marked by the arrow represent?

Answer 1

I think Greg Martin's suggestion that it is a $\ne$ sign is clearly correct. The slash in the $\ne$ has been mistakenly rendered as a rectangle instead. As I mentioned, this is a typical sort of error in computer typesetting.

If it were $\ne$, the mathematics would be (almost) correct. For example, in the third bullet point, there are two cases: a circle if $A=C$, and an ellipse if $A\ne C$. It appears to be discussing the discriminant of a conic section; compare Wikipedia's treatment.

I did not pick up on this sooner because the first line would say $\Delta\ne 0$ but then in the second bullet point, $B^2-4AC=0$ would imply $\Delta=0$. I guess that this is an editing error.