Đáp án:
Giải thích các bước giải:
\(\begin{array}{l}
a.\overrightarrow {AM} = \overrightarrow {SM} - = \frac{1}{2}\overrightarrow {SD} - \overrightarrow {SA} \\
= \frac{1}{2}\overrightarrow {SC} + \frac{1}{2}\overrightarrow {CD} - \overrightarrow {SA} = \frac{1}{2}\overrightarrow {SC} + \frac{1}{2}\overrightarrow {BA} - \overrightarrow {SA} \\
= \frac{1}{2}\overrightarrow {SC} + \frac{1}{2}\overrightarrow {SA} - \frac{1}{2}\overrightarrow {SB} - \overrightarrow {SA} \\
= \frac{1}{2}\overrightarrow {SC} - \frac{1}{2}\overrightarrow {SB} - \frac{1}{2}\overrightarrow {SA}
\end{array}\)
