题目

I am here to announce my retirement from the game of basketball. It won't be another announcement to baseball or anything to that nature. Mentally, I'm exhausted; I don't feel I have a challenge. Physically, I feel great. The last time in 1993 I had other agendas. I felt that I wanted to play baseball and I felt that at my age, it was a good opportunity and time to do it. And with the death of my father, and I was basically trying to deal with that. Actually I talked to Jerry last year once the season ended and I told Jerry at that time, mentally, I was a little exhausted. I didn't kno（本小题满分12分）如图，四棱锥的底面是正方形，平面，，，点是上的点，且．（1）求证：对任意的，都有．（2）设二面角的大小为，直线与平面所成的角为，若，求的值．