题目

We can read the time 3: 45 as . A . a quarter to three B . fifteen past three C . a quarter to four D . a quarter past three 答案：C18．如图在三棱柱ABC-A1B1C1中，已知AB⊥侧面BB1C1C，BC=$\sqrt{2}$，AB=CC1=2，∠BCC1=$\frac{π}{4}$，点E在棱BB1上．（1）求C1B的长，并证明C1B⊥平面ABC；（2）若BE=λBB1，试确定λ的值，使得二面角A-C1E-C的余弦值为$\frac{\sqrt{5}}{5}$．