题目

—We _____ talk loudly when we see the sign “Be quiet” on the wall. —Yes, you are right. We should be polite in public. A. must          B. mustn’t         C. need                D. needn’t 答案：B已知长方体ABCD-A′B′C′D′，AB=2，AA′=1，直线BD与平面AA′B′B所成角为30°，E为A′B′的中点．（1）求异面直线AC与BE所成的角；（2）求A点到平面BDE的距离．