## Precalculus (6th Edition) Blitzer

$\{x\ |\ \ x \lt 5.5 \ \}$
Recall the general rules for writing intervals: An interval may be annotated as $[a,b],\ (a,b],\ [a,b),\ (a,b),\ (-\infty,b],\ (-\infty,b),\ [a,\infty),\ (a,\infty).$ It contains numbers between the left and right borders. The inequality signs depend on whether a border is included in the set or not. The bracket "[", or "]" means "border included" and the sign is "$\leq$". The parenthesis "(" or ")" means "border excluded" and the sign is "$\lt$". $\pm\infty$ implies "no border", so it is always accompanied by a parenthesis. For example $[a,b)=\{x\ |\ a\leq x\lt b\ \}$ $(-6,\infty)=\{x\ |\ x\gt-6\ \}$ $(-\infty,3]=\{x\ |\ \ x\leq 3\ \}$ --- Here, the left border is unbounded. and the right border is excluded. We write this as $-\infty \lt x \lt 5.5$ or, simply as: $\qquad x \lt 5.5.$ $(-\infty,5.5)$ = $\{x\ |\ \ x \lt 5.5 \ \}$