a) \(\overrightarrow{MN}=\overrightarrow{MA}+\overrightarrow{AD}+\overrightarrow{DN}.\)
\(\overrightarrow{MN}=\overrightarrow{MB}+\overrightarrow{BC}+\overrightarrow{CN}.\)
Cộng từng vế ta được: \(\overrightarrow{MN}=\dfrac{1}{2}\left ( \overrightarrow{AD}+\overrightarrow{BC} \right )\)
b)
\(\eqalign{
& \overrightarrow {MN} = \overrightarrow {MA} + \overrightarrow {AC} + \overrightarrow {CN} \cr
& \overrightarrow {MN} = \overrightarrow {MB} + \overrightarrow {BD} + \overrightarrow {DN} \cr} \)
Cộng từng vế ta được: \(\overrightarrow{MN}=\dfrac{1}{2}\left ( \overrightarrow{AC}+\overrightarrow{BD} \right ).\)