Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
a,\\
\overrightarrow {AD} + \overrightarrow {BC} = \left( {\overrightarrow {AM} + \overrightarrow {MD} } \right) + \left( {\overrightarrow {BM} + \overrightarrow {MC} } \right)\\
= \left( {\overrightarrow {AM} + \overrightarrow {BM} } \right) + \left( {\overrightarrow {MD} + \overrightarrow {MC} } \right)\\
= \overrightarrow 0 + 2.\overrightarrow {MN} \\
= 2\overrightarrow {MN} = \overrightarrow {MK} \\
b.\\
\overrightarrow {AC} + \overrightarrow {BD} = \left( {\overrightarrow {AD} + \overrightarrow {DC} } \right) + \left( {\overrightarrow {BC} + \overrightarrow {CD} } \right)\\
= \left( {\overrightarrow {AD} + \overrightarrow {BC} } \right) + \left( {\overrightarrow {DC} + \overrightarrow {CD} } \right)\\
= \overrightarrow {MK} + \overrightarrow 0 \\
= \overrightarrow {MK}
\end{array}\)
