## Precalculus (6th Edition) Blitzer

An interval may be annotated as $[a,b],\ (a,b],\ [a,b),\ (a,b),\ (-\infty,b],\ (-\infty,b),\ [a, \infty),\ (a, \infty).$ It indicates 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 $[a,b)$ = $\{x\ |\; a\leq x \lt b\ \ \}$ $(-6,\infty)$ = $\{x\ |\; x \gt -6\ \ \}$ $(-\infty,3] = \{x\ |\; x \leq 3\ \ \}$ --- "$(-2$..." means that the left border 2 is excluded, "... $\infty)$" means that there is no right border. The interval contains numbers x such that $-2 \lt x \lt \infty$ which are numbers greater than $-2.$