题目

His red clothes stood out clearly _____ the white and shinny snow. A . across B . against C . beyond D . beneath 答案：B如图，是边长为3的正方形，，，与平面所成的角为.(1)求二面角的的余弦值;(2)设点是线段上一动点，试确定的位置，使得，并证明你的结论．