Cho \(\Delta\)ABC có G là trọng tâm. Gọi D, E thỏa \(2\overrightarrow{CD}=3\overrightarrow{DB}\), \(5\overrightarrow{EB}=2\overrightarrow{EC}\).
a/ Tính \(\overrightarrow{AD},\overrightarrow{AE}\) theo\(\overrightarrow{AB},\overrightarrow{AC}\)
b/ Tính \(\overrightarrow{AG}\) theo \(\overrightarrow{AD},\overrightarrow{AE}\)
\(\text{a) Ta có : }2\overrightarrow{CD}=3\overrightarrow{DB}\\ \Rightarrow\overrightarrow{DC}=-\frac{3}{2}\overrightarrow{DB}\\ \Rightarrow D;B;C\text{ thẳng hàng },D\text{ nằm giữa }B;C\left(\frac{3}{2}< 0\right)\\ \Rightarrow\overrightarrow{BC}=\overrightarrow{BD}+\overrightarrow{DC}=\overrightarrow{BD}+\frac{3}{2}\overrightarrow{BD}=\frac{5}{2}\overrightarrow{BD}\\ 5\overrightarrow{EB}=2\overrightarrow{EC}\\ \Rightarrow\overrightarrow{EB}=\frac{2}{5}\overrightarrow{EC}\\ \Rightarrow E;B;C\text{ thẳng hàng },B\text{ nằm giữa }E;C\left(\frac{2}{5}>0;EB< EC\right)\\ \Rightarrow\overrightarrow{BC}=\overrightarrow{EC}-\overrightarrow{EB}=\overrightarrow{EC}-\frac{2}{5}\overrightarrow{EC}=\frac{3}{5}\overrightarrow{EC}\)
\(\Rightarrow\overrightarrow{AD}=\overrightarrow{AB}+\overrightarrow{BD}\\ =\overrightarrow{AB}+\frac{2}{5}\overrightarrow{BC}=\overrightarrow{AB}+\frac{2}{5}\left(\overrightarrow{AC}-\overrightarrow{AB}\right)\\ =\overrightarrow{AB}+\frac{2}{5}\overrightarrow{AC}-\frac{2}{5}\overrightarrow{AB}=\frac{3}{5}\overrightarrow{AB}+\frac{2}{5}\overrightarrow{AC}\)
\(\overrightarrow{AE}=\overrightarrow{EC}+\overrightarrow{CA}\\ =\frac{5}{3}\overrightarrow{BC}-\overrightarrow{AC} =\frac{5}{3}\left(\overrightarrow{AC}-\overrightarrow{AB}\right)-\overrightarrow{AC}\\ =\frac{5}{3}\overrightarrow{AC}-\frac{5}{3}\overrightarrow{AB}-\overrightarrow{AC}=\frac{2}{3}\overrightarrow{AC}-\frac{5}{3}\overrightarrow{AB}\)
\(b\text{) Theo tính chất trọng tâm }\Delta:3\overrightarrow{AG}=\overrightarrow{AA}+\overrightarrow{AB}+\overrightarrow{AC}\\ =\overrightarrow{0}+\overrightarrow{AB}+\overrightarrow{AC}\\ =\left(\frac{9}{4}\overrightarrow{AB}+\frac{3}{2}\overrightarrow{AC}\right)-\left(\frac{1}{2}\overrightarrow{AC}+\frac{5}{4}\overrightarrow{AC}\right)\\ =\frac{15}{4}\left(\frac{3}{5}\overrightarrow{AB}+\frac{2}{5}\overrightarrow{AC}\right)-\frac{3}{4}\left(\frac{2}{3}\overrightarrow{AC}+\frac{5}{3}\overrightarrow{AC}\right)\\ =\frac{15}{4}\overrightarrow{AD}-\frac{3}{4}\overrightarrow{AE}\)
