题目

I received a call today asking if I would be willing to bring food to a family in need. The mother was having a major operation and would be lying down for several weeks. Of course, I responded with an immediate “Yes！”. As I planned the meal in my head, I reflected on how many times over the years I had been asked to prepare food. I have done so countless times with a very open heart. But the truly amazing thing is that I have received double over the course of my life. When my mother passed away, our house was filled with fresh dinners for weeks. A woman from the church of our c（本小题满分13分）如图，四棱锥S-ABCD的底面是正方形，SD⊥平面ABCD．SD=2，，E是SD上的点．（Ⅰ）求证：AC⊥BE；（Ⅱ）求二面角C—AS—D的余弦值．