题目

如图，四棱锥中，面，，，，.（1）证明：平面；（2）若为中点，求二面角的余弦值. 答案：【答案】（1）证明见解析；（2）.【解析】（1）由平面可得出，再由得出，再利用线面垂直的判定定理可得出结论；（2）计算出，然后以点为坐标原点，以、、过点且垂直于的直线分别为、、轴建立空间直角坐标系，利用空间向量法可计算出二面角的余弦值.（1）平面，平面，，，，，平面；（2）取 —I'm sorry, I shouldn't have been so rude to you. —You ________ your temper but that's OK. A. have lost　        B. had lost            C. did lose　      D. were losing