• March 16th 2013, 02:47 PM
soso123
• March 16th 2013, 05:13 PM
Plato
$\overrightarrow {AB} = \overline {AB} \cup \left\{ {X:A - B - X} \right\}$.
Now combine that with $\overrightarrow {BA}$ to get the set $\overleftrightarrow {AB}$.