题目

I was very nervous because it was the first time that I (invite) to an opening banquet. 答案：【1】had been invited16．如图，梯形ABEF中，AF∥BE，AB⊥AF，且AB=BC=AD=DF=2CE=2，沿DC将梯形CDFE折起，使得平面CDFE⊥平面ABCD．（1）证明：AC∥平面BEF；（2）求三棱锥D-BEF的体积．