$\displaystyle{\stackrel{{\leftrightarrow}}{{{C}{D}}}}\usepackage{cmbright}\Huge\$ and $\displaystyle{\stackrel{{\leftrightarrow}}{{{E}{F}}}}\usepackage{cmbright}\Huge\$ are not perpendicular. They do not intersect at a $\displaystyle{{90}}^{\circ}\usepackage{cmbright}\Huge\$ angle, because they do not intersect at all!