题目

________, I think, tired as you are, and the problem could be settled. A. Making great efforts                     B. So long as you keep up your spirits C. If you double your efforts            D. A bit more efforts 答案：D如图，抛物线y=-x2+bx+2交x轴于A、B两点（点B在点A的左侧），交y轴于点C，其对称轴为x=，O为坐标原点．（1）求A、B、C三点的坐标；（2）求证：∠ACB是直角；（3）抛物线上是否存在点P，使得∠APB为锐角？若存在，求出点P的横坐标的取值范围；若不存在，请说明理由．