Question

If the product of the two slopes of two lines equals -1, then the lines are ?

Answer 1

$\bf \stackrel{\textit{perpendicular lines have \underline{negative reciprocal} slopes}} {\stackrel{slope}{\cfrac{a}{b}}\qquad \qquad \qquad \stackrel{reciprocal}{\cfrac{b}{a}}\qquad \stackrel{negative~reciprocal}{-\cfrac{b}{a}}} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ ~\hfill \left( \cfrac{a}{b} \right)\left( -\cfrac{b}{a} \right)=-1~\hfill$