28.(2023•河北模拟)已知OOO为ΔABC\Delta ABCΔABC的外心,若OA=2OA=2OA=2,且∠BAC=75∘\angle BAC=75\circ∠BAC=75∘,则OB→⋅OC→=\overrightarrow{OB}\cdot \overrightarrow{OC}=OB⋅OC=___−23-2\sqrt{3}−23___.