Appendix A - Appendix A Exercises - Page 1158: 14

$${\text{Their slopes are negative reciprocals.}}$$

$$\eqalign{ & {\text{Two lines }}{l_1}{\text{ and }}{l_2}{\text{ with slopes }}{m_1}{\text{ and }}{m_2,}{\text{ respectively, are}} \cr & {\text{perpendicular if and only if}},{\text{ }}{m_1}{m_2} = - 1,{\text{ that is}},{\text{ their slopes }} \cr & {\text{are negative reciprocals}}: \cr & {m_1} = - \frac{1}{{{m_2.}}} \cr} $$