题目

Violence (暴力) at schools has been widely reported in recent years. Hitting, slapping (打耳光) and burning others with cigarettes－the violence among teenagers has drawn public and government attention. On Nov 11, the government published a guideline (指导意见) on dealing with school violence. The guideline stressed that students with serious behaviour problems should be taken to special schools. Or, in more serious cases, they may take criminal responsibility (刑事责任) and be sent to prison. China has long been considered as too soft on teen offenders (犯事者), accord4．如图，圆柱的轴截面ABCD是正方形，点E在底面的圆周上，BF⊥AE，F是垂足．（1）求证：BF⊥AC；（2）如果圆柱与三棱锥A-BCE的体积比等于3π，求二面角B-AC-E的余弦值．