[กิตติ]

$\overrightarrow{a} +\overrightarrow{b} +\overrightarrow{c} =\overrightarrow{0} $
โจทย์ถาม $\overrightarrow{a} \bullet \overrightarrow{b} +\overrightarrow{b}\bullet \overrightarrow{c}$
$\overrightarrow{a} \bullet \overrightarrow{b} +\overrightarrow{b}\bullet \overrightarrow{c}$
$=\overrightarrow{a} \bullet \overrightarrow{b} +\overrightarrow{c}\bullet \overrightarrow{b}$
$=\left(\,\overrightarrow{a} +\overrightarrow{c}\right)\bullet \overrightarrow{b}$
$=\left(\,-\overrightarrow{b}\right) \bullet\overrightarrow{b}$
$=-\left(\,\overrightarrow{b}\bullet\overrightarrow{b}\right) $
$=-\left|\,\overrightarrow{b}\right|^2 $
$=-4$