[tex]\it 5)\ \overline{AM} = \overline{AC}+\overline{CM}=\overline{AC}-\overline{MC}=\overline{AC}-\dfrac{3}{4}\overline{BC}=\overline{AC}-\dfrac{3}{4}(\overline{AC}-\overline{AB})=\\ \\ =\overline{AC}-\dfrac{3}{4}\overline{AC}+\dfrac{3}{4}\overline{AB}=\dfrac{3}{4}\overline{AB}+\dfrac{1}{4}\overline{AC}=\dfrac{3}{4}\overline{AB}-\dfrac{1}{4}\overline{CA}[/tex]
