题目

已知三棱锥的各棱长都相等，为中点，则异面直线与所成角的余弦值为(    ) A.                B.                 C.                D. 答案：B 【解析】求线面角关键找平行，利用三角形中位线是解决本题的关键，再根据余弦定理求求得结果. 详解：取AC中点M，则因为为中点，因此ME平行AB，从而异面直线与所成角等于∠MED，因为三棱锥的各棱长都相等，设为1，则 ，即异面直线与所成角的余弦值为，选B. 点睛：本题主要考查求异面直线所成角用所给词的适当形式填空。1. _______ (keep) safe, we must look out of the car.2. Look, the children ______ (cross) the road with their teachers.3. Mr. Smith ______ (take) me to the classroom last Monday.4. We mustn’t run _______ (quick) on the road.5. I’d like _____________ (go) the bus station with you.