Giải thích các bước giải:
$\begin{array}{l}
a)\overrightarrow {DC} + 2\overrightarrow {BM} \\
= \overrightarrow {DC} + \overrightarrow {BA} \\
= \overrightarrow {DC} + \overrightarrow {CD} \\
= \overrightarrow {DD} \\
= \overrightarrow 0 \\
b)\overrightarrow {OA} + \overrightarrow {OB} + \overrightarrow {OC} + \overrightarrow {OD} \\
= \left( {\overrightarrow {OA} + \overrightarrow {OC} } \right) + \left( {\overrightarrow {OB} + \overrightarrow {OD} } \right)\\
= \overrightarrow 0 + \overrightarrow 0 \\
= \overrightarrow 0 \\
c)2\overrightarrow {OM} + \dfrac{1}{2}\overrightarrow {AC} + \overrightarrow {OD} \\
= \overrightarrow {OA} + \overrightarrow {OB} + \overrightarrow {AO} + \overrightarrow {OD} \\
= \left( {\overrightarrow {OA} + \overrightarrow {AO} } \right) + \left( {\overrightarrow {OB} + \overrightarrow {OD} } \right)\\
= \overrightarrow 0 + \overrightarrow 0 \\
= \overrightarrow 0 \\
d)\overrightarrow {AB} + \overrightarrow {CM} - \left( {\overrightarrow {AM} + \overrightarrow {CB} } \right)\\
= \left( {\overrightarrow {AB} - \overrightarrow {AM} } \right) + \left( {\overrightarrow {CM} - \overrightarrow {CB} } \right)\\
= \overrightarrow {MB} + \overrightarrow {BM} \\
= \overrightarrow {MM} \\
= \overrightarrow 0 \\
\Rightarrow \overrightarrow {AB} + \overrightarrow {CM} - \left( {\overrightarrow {AM} + \overrightarrow {CB} } \right) = \overrightarrow 0 \\
\Rightarrow \overrightarrow {AB} + \overrightarrow {CM} = \overrightarrow {AM} + \overrightarrow {CB}
\end{array}$
