$\displaystyle{\stackrel{{\leftrightarrow}}{{{C}{D}}}}\usepackage{cmbright}\Huge\$ and $\displaystyle{\stackrel{{\leftrightarrow}}{{{E}{F}}}}\usepackage{cmbright}\Huge\$ are both perpendicular to $\displaystyle{\stackrel{{\leftrightarrow}}{{{A}{B}}}}\usepackage{cmbright}\Huge\$ . When two lines are perpendicular to a third line, they are parallel to each other.