5)
[tex]\it \overline{AM} = \overline{AB}+\overline{BM}= \overline{AB}+\dfrac{1}{3}\overline{BC}=\overline{AB}+\dfrac{1}{3}(\overline{AC}-\overline{AB})=\dfrac{2}{3}\overline{AB}+\dfrac{1}{3}\overline{AC}[/tex]
[tex]\it 6)\ a\in\Big(\dfrac{\pi}{2},\ \pi\Big) \Rightarrow cosa<0\\ \\ cosa=-\sqrt{1-sin^2a} =-\sqrt{1-\dfrac{9}{25}}=-\sqrt{\dfrac{16}{25}}=-\dfrac{4}{5}\\ \\ \\ tga=\dfrac{sina}{cosa} =\dfrac{\dfrac{3}{5}}{-\dfrac{4}{5}}=-\dfrac{3}{5}:\dfrac{4}{5}=-\dfrac{3}{5}\cdot\dfrac{5}{4}=-\dfrac{3}{4}[/tex]
