爸爸认为行,但你必须先完成作业,他这样说:
Sure, you ____. But you have to ____ ____ ____first.
O为平面上的定点,A、B、C是平面上不共线的三点,若(OA→﹣OC→)•(OA→+OC→﹣2OB→)=0,则△ABC是( )