Áp dụng quy tắc hình bình hành ta có:
\(\left\{{}\begin{matrix}\overrightarrow{AB}+\overrightarrow{AD}=\widehat{AC}\\\overrightarrow{AD}-\overrightarrow{AB}=\overrightarrow{BD}\end{matrix}\right.\)
Từ hệ trên suy ra:
\(\overrightarrow{2AB}=\left(\overrightarrow{AB}+\overrightarrow{AD}\right)-\left(\overrightarrow{AD}-\overrightarrow{AB}\right)=\overrightarrow{AC}-\overrightarrow{BD}\)
\(\Leftrightarrow\overrightarrow{AB}=\frac{1}{2}\left(\overrightarrow{AC}-\overrightarrow{BD}\right)=\frac{1}{2}\left[7-\left(-3\right);-3-4\right]=\left(5;\frac{-7}{2}\right)\)
