题目

Jenny hopes that Mr. Smith will suggest a good way to have her written English ____ in a short period. A． improved     B． improving   C． to improve     D． improve 答案：A如图，在五棱锥P-ABCDE中，PA=AB=AE=2a，PB=PE=，BC=DE=a，∠EAB=∠ABC=∠DEA=90°．（1）求证：PA⊥平面ABCDE；（2）求异面直线CD与PB所成角的大小；（3）求二面角A-PD-E的大小．