(Đúng hay sai) 2MN = AB + CD

d)Chọn sai

\(2\overrightarrow {NM}  = \overrightarrow {AB}  + \overrightarrow {CD} \) sai vì:

\[\begin{array}{l}\overrightarrow {AB}  + \overrightarrow {CD}  = \left( {\overrightarrow {AM}  + \overrightarrow {MN}  + \overrightarrow {NB} } \right) + \left( {\overrightarrow {CM}  + \overrightarrow {MN}  + \overrightarrow {ND} } \right)\\ = 2\overrightarrow {MN}  + \left( {\overrightarrow {AM}  + \overrightarrow {CM} } \right) + \left( {\overrightarrow {NB}  + \overrightarrow {ND} } \right) = 2\overrightarrow {MN}  + \overrightarrow 0  + \overrightarrow 0  = 2\overrightarrow {MN} .\end{array}\]