题目

t has been raining for a day, but even though it hadn’t rained, we _____ there by tomorrow. A. can’t get       B. won’t get        C. hadn’t got      D. wouldn’t get   答案：Awe can’t get there by tomorrow 为客观事实，故宜用陈述语气。如图，在斜三棱柱中，O是AC的中点，平面，，.（1）求证：平面；（2）求二面角的余弦值.