[tex]\it \left.\begin{aligned}\overline{PA}\ =\ -\dfrac{1}{4}\overline{AB}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \\ \overline{PB}=\dfrac{3}{4}\overline{AB}|_{\cdot\dfrac{1}{3}}\ \Rightarrow \dfrac{1}{3}\overline{PB}=\dfrac{1}{4}\overline{AB}\ \end{aligned}\right\}\ \Rightarrow\ \overline{PA}+\dfrac{1}{3}\overline{PB}=-\dfrac{1}{4}\overline{AB}+\dfrac{1}{4}\overline{AB}=\overline{0}[/tex]
