Answer
$\{x\ |\ \ x \lt 3 \ \}$
Work Step by Step
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 with which we write the inequalities for an interval depend on whether a border is included in the set or not.
The bracket "[", or "]" means "border included" $\Rightarrow $ the sign is "$\leq $".
The parenthesis "(" or ")" means "border excluded" $\Rightarrow $ the sign is "$\lt $".
$\pm\infty $ implies "no bound" and is always accompanied by a parenthesis.
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Here, the left border is unbounded and the right border is included.
We write this as
$-\infty \lt x \lt 3$
or, simply as $\qquad x \lt 3.$
$(-\infty,3)$ = $\{x\ |\ \ x \lt 3 \ \}$
