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

$-r = t-s$
We use the equation for slope: $m = \frac{y_2-y_1}{x_2-x_1}$ We set the two slopes equal to each other, for they are parallel: $\frac{v}{-r} = \frac{v}{t-s} \\ \frac{1}{-r} = \frac{1}{t-s} \\ -r = t-s$