Theo quy tắc hình bình hành ta có: \(\overrightarrow {AB} + \overrightarrow {AD} = \overrightarrow {AC} .\)
\(\Rightarrow \overrightarrow{AB} + \overrightarrow{AC}+ \overrightarrow{AD}= \overrightarrow{AB} + \overrightarrow{AD}\)\(+ \overrightarrow{AC}\)
\(\Rightarrow \overrightarrow{AB}+ \overrightarrow{AC} + \overrightarrow{AD}= \overrightarrow{AC} +\overrightarrow{AC} \)\(=2\overrightarrow{AC}.\)