Here's the set:
$\{x:x<8\} \setminus \{5\}$
What does the "\" operator mean?
Here's the set:
$\{x:x<8\} \setminus \{5\}$
What does the "\" operator mean?
See http://en.wikipedia.org/wiki/Complement_%28set_theory%29
for "relative complement", "set-theoretic difference".
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
In general, $A \backslash B = A \cap B^c$
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.