题目

Have you ever heard of a ball of energy? People often use it to describe very active children. But today we tell about an invention called the soccket, that is a real soccer ball of energy. Julia Silverman explains that in fact the soccket is a portable generator (便携式发电机).Julin Silverman and Jessica Matthews developed the soccket as part of a group project for an engineering class at Harvard University.There are mechanisms (装置) in a soccket. When you kick, hit or throw it, energy is then kept in it by these mechanisms instead of disappearing into the environment.Then the user c20．已知△ABC是边长为2的正三角形，点P为△ABC内一点，且$\overrightarrow{PA}$+2$\overrightarrow{PB}$+3$\overrightarrow{PC}$=0，则$\overrightarrow{PA}$•$\overrightarrow{PB}$=-$\frac{2}{9}$．