已知矩形ABCD的长AB=4，宽AD=3，将其沿对角线BD折起，得到四面体A﹣BCD，如图所示，给出下列结论：

①四面体A﹣BCD体积的最大值为 $\frac{72}{5}$ ；

②四面体A﹣BCD外接球的表面积恒为定值；

③若E、F分别为棱AC、BD的中点，则恒有EF⊥AC且EF⊥BD；

④当二面角A﹣BD﹣C为直二面角时，直线AB、CD所成角的余弦值为 $\frac{16}{25}$ ；

⑤当二面角A﹣BD﹣C的大小为60°时，棱AC的长为 $\frac{14}{5}$ ．

其中正确的结论有____（请写出所有正确结论的序号）．

  以下内容摘自小明的微博：

2月17号		2月26号
刚开学老师就批评我，不就是上课玩手机还有去了两回黑网吧嘛！	小明的微博	超市老板竟然卖给我过期的面包，去找他换，可他竟然不承认，只能自认倒霉了。
3月10号		3月13号：
骗老妈多要点钱被她发现，结果骂了我一顿，烦死了！以后她说什么我也不听她的。		今天的英语口语模拟考试我竟然不及格！我觉得自己太差劲了！不过秦川这小子竟然考得比我还低，我也好好笑话了他一下，哈哈！

Once upon a time, there lived a rich man. He had a servant (仆人). He and the servant loved wine and good food very much. Each time the rich man left his home, the servant would drink the wine and eat up all the nice food in the house. The rich man knew what his servant did, but he had never caught his servant doing that.
　　One morning, when he left home, he said to the servant, "Here are two bottles of poison (毒药) and some nice food in the house. You must take good care of them. " With these words, he went out.
　　But the servant knew that the rich man had said was untrue. After the rich man was away from his home, he enjoyed a nice meal. Because he drank too much, he was drunk and fell to the ground. When the rich man came back, he couldn't find his food and his wine. He became very angry. He woke the servant up. But the servant told his story very well. He said a cat had eaten up everything. He was afraid to be punished（惩罚）, so he drank the poison to kill himself.