## Geometry: Common Core (15th Edition)

Slope of AB: $m=\frac{11-3}{-4--8}$ $m=2$ Slope of CD: $m=\frac{3-2}{-1-1}$ $m=-\frac{1}{2}$ Because $-\frac{1}{2}$ is the negative recipricol of 2, lines AB and CD are perpendicular.