We use solid lines whenever the inequality contains [tex]\leq[/tex] or [tex]\geq[/tex]. That means it contains an equal sign which means the boundary line is inclusive.
On the other hand when the inequality does not have an equal sign in it we use a dashed line to indicate that all values on the boundary line are not inclusive.
That is we use a dashed or broken line for the boundary line of an inequality whenever it contains [tex]\:<\: or \:>\:[/tex].
