## Elementary Geometry for College Students (6th Edition)

$\frac{a-c}{e-c} =-1$
For the two lines to be perpendicular, slopes must be opposite reciprocals. We use the equation for slope: $m_1 =\frac{b-d}{a-c}$ $m_2 = \frac{b-d}{e-c}$ The reciprocal of $m_1$ times $m_2$ must equal -1. Thus, we find the equation: $\frac{a-c}{b-d}\frac{b-d}{e-c} = -1 \\ \frac{a-c}{e-c} =-1$