Bài 2:
M thuộc tia đối của tia BC
=>B nằm giữa M và C
=>MC=MB+BC=1/2BC+BC=3/2BC
\(\overrightarrow{AM}=\overrightarrow{AB}+\overrightarrow{BM}\)
\(=\overrightarrow{AB}-\dfrac{1}{2}\overrightarrow{BC}\)
\(=\overrightarrow{AB}-\dfrac{1}{2}\left(\overrightarrow{BA}+\overrightarrow{AC}\right)\)
\(=\overrightarrow{AB}-\dfrac{1}{2}\overrightarrow{BA}-\dfrac{1}{2}\overrightarrow{AC}=\dfrac{3}{2}\overrightarrow{AB}-\dfrac{1}{2}\overrightarrow{AC}\)
Bài 1:
MB=2MC
=>\(BM=\dfrac{2}{3}BC\)
\(\overrightarrow{AM}=\overrightarrow{AB}+\overrightarrow{BM}=\overrightarrow{AB}+\dfrac{2}{3}\overrightarrow{BC}\)
\(=\overrightarrow{AB}+\dfrac{2}{3}\left(\overrightarrow{BA}+\overrightarrow{AC}\right)\)
\(=\overrightarrow{AB}+\dfrac{2}{3}\overrightarrow{BA}+\dfrac{2}{3}\overrightarrow{AC}=\dfrac{1}{3}\overrightarrow{AB}+\dfrac{2}{3}\overrightarrow{AC}\)
