[tex]\bf \stackrel{\textit{perpendicular lines have \underline{negative reciprocal} slopes}}
{\stackrel{slope}{-\cfrac{1}{9}}\qquad \qquad \qquad \stackrel{reciprocal}{-\cfrac{9}{1}}\qquad \stackrel{negative~reciprocal}{+\cfrac{9}{1}}\implies 9}[/tex]
