Theo bài ra: ABCD là hình bình hành.
=> $\overrightarrow{AB}= \overrightarrow{DC}$
=> $\overrightarrow{AB}+ \overrightarrow{AC}+\overrightarrow{AD} = \overrightarrow{AD}+\overrightarrow{DC}+\overrightarrow{AC}$
<=> $\overrightarrow{AB}+ \overrightarrow{AC}+\overrightarrow{AD}=\overrightarrow{AC}+\overrightarrow{AC}=2\overrightarrow{AC}$ (đpcm)