Vecto AB- vecto CD =vectoAC- vecto BD

\(\begin{array}{l}
\overrightarrow {AB}  - \overrightarrow {CD}  = \overrightarrow {AC}  - \overrightarrow {BD} \\
 \Leftrightarrow \overrightarrow {AB}  - \overrightarrow {AC}  = \overrightarrow {CD}  + \overrightarrow {DB} \\
 \Leftrightarrow \overrightarrow {CB}  = \overrightarrow {CB} \,\,\left( {ld} \right) \Rightarrow dpcm
\end{array}\)

$\vec{AB} - \vec{AC}=\vec{CB}$

$\vec{CD}-\vec{BD}=\vec{CD}+\vec{DB}=\vec{CB}$

$\Rightarrow \vec{AB}-\vec{AC}=\vec{CD}-\vec{BD}$

$\Leftrightarrow \vec{AB}-\vec{CD}=\vec{AC}-\vec{BD}$