Cho 6 điểm abcdef cmr ad-fc-eb=cd-ea-fb

AD−FC−−−→EB=−−→CD−−−→EA−−−→FB⇔−−→AD+−−→CF+−−→BE+−−→DC+−−→EA+−−→FB=→0⇔(−−→AD+−−→DC)+(−−→CF+−−→FB)+(−−→BE+−−→EA)=→0⇔−−→AC+−−→CB+−−→BA=→0⇔−−→AB+−−→BA=→0(ld)

\(\begin{array}{l}
\overrightarrow {AD}  - \overrightarrow {FC}  - \overrightarrow {EB}  = \overrightarrow {CD}  - \overrightarrow {EA}  - \overrightarrow {FB} \\
 \Leftrightarrow \overrightarrow {AD}  + \overrightarrow {CF}  + \overrightarrow {BE}  + \overrightarrow {DC}  + \overrightarrow {EA}  + \overrightarrow {FB}  = \overrightarrow 0 \\
 \Leftrightarrow \left( {\overrightarrow {AD}  + \overrightarrow {DC} } \right) + \left( {\overrightarrow {CF}  + \overrightarrow {FB} } \right) + \left( {\overrightarrow {BE}  + \overrightarrow {EA} } \right) = \overrightarrow 0 \\
 \Leftrightarrow \overrightarrow {AC}  + \overrightarrow {CB}  + \overrightarrow {BA}  = \overrightarrow 0 \\
 \Leftrightarrow \overrightarrow {AB}  + \overrightarrow {BA}  = \overrightarrow 0 \left( {ld} \right)
\end{array}\)