题目

For almost two months Dominic York, a 23-year-old hairdresser, wandered about hospitals all night, wearing a white coat and pretending he was a doctor. Yesterday he proudly claimed in court that despite his complete lack of medical experience or qualifications, he had saved several people’s lives. He had even been allowed to assist a surgeon during an emergency operation on a patient who was about to die on something she had swallowed.    “I watched one of those TV dramas about a hospital and suddenly I felt like playing one of the roles myself. So I put on a white jacket and a stethosco如下图所示，O是四边形ABCD内任一点，请根据图中给出的向量，确定a、b、c、d的方向（用箭头表示），使a+b=,c-d=,并画出b-c和a+d．