Unfamiliar Set operation Here's the set:
$$\{x:x<8\} \setminus \{5\}$$
What does the "\" operator mean?
 A: See
http://en.wikipedia.org/wiki/Complement_%28set_theory%29
for "relative complement", "set-theoretic difference".
A: It means that you are looking at all values less than $8$ without considering $5$. What you have written is not completely correct. If you are talking of real numbers less than $8$ and not equal to $5$ you will write it as $$\{x \in \mathbb{R}:x<8 \} \backslash \{5\} = \{x \in \mathbb{R}:x<5 \} \cup \{x \in \mathbb{R}:5<x<8 \} = (-\infty,5) \cup (5,8)$$ or if you are talking of natural numbers less than $8$ and not equal to $5$ you will write it as $$\{x \in \mathbb{N}:x<8 \} \backslash \{5\} = \{0,1,2,3,4,6,7\}$$ or if you are talking of integers less than $8$ and not equal to $5$ you will write it as $$\{x \in \mathbb{Z}:x<8 \} \backslash \{5\} = \{x \in \mathbb{Z}:x<5 \} \cup \{6,7\}$$
It is incorrect to write $$\{x:x<8 \} \backslash \{5\}$$ without specifying where $x$ belongs
In general, $A \backslash B = A \cap B^c$
A: Intuitively similar (but not really) to minus. So if $A=\{1,2,4\}$ and $B=A\setminus\{2\}$, then $B=\{1,4\}$.
in your case, the set is $\{x:x<8, x\neq5\}$.
Look at the other answers wiki link.
