17.(2023•浑南区校级模拟)在ΔABC\Delta ABCΔABC中,B=60∘B=60\circB=60∘,OOO是ΔABC\Delta ABCΔABC的外心,若OB=2OB=2OB=2,则AO→⋅AC→=(\overrightarrow{AO}\cdot \overrightarrow{AC}=(AO⋅AC=( )))