题目

She was working in the room when she heard a boy crying and ________ out as quickly as she could to see what was happening. A. going          B. go         C. had gone         D. went 答案：D13．如图四边形PABC中，∠PAC=∠ABC=90°，$PA=AB=2\sqrt{3}，AC=4$，现把△PAC沿AC折起，使PA与平面ABC成60°，设此时P在平面ABC上的投影为O点（O与B在AC的同侧），（1）求证：OB∥平面PAC；（2）求二面角P-BC-A大小的正切值．