8. Given $A, B, C$ are three points on $\odot O$, and $\overrightarrow{A O}=\frac{1}{2}(\overrightarrow{A B}+\overrightarrow{A C})$, then the dot product $\overrightarrow{A B} \cdot \overrightarrow{A C}=$
Since $O$ is the midpoint of $BC$, then $AB \perp AC \Rightarrow \overrightarrow{AB} \cdot \overrightarrow{AC}=0$.
