9. 已知正方形ABCDABCDABCD的边长为2，点PPP满足AP→=12(AB→+AC→)\overrightarrow{AP}=\frac{1}{2}(\overrightarrow{AB}+\overrightarrow{AC})AP=21(AB+AC)，则∣PD→∣=\vert \overrightarrow{PD}\vert =∣PD∣= ；PB→⋅PD→=\overrightarrow{PB}\cdot \overrightarrow{PD}=PB⋅PD= ．