题目

On morning Mrs Peter said to her husband, “Jack, there is a meeting of our ladies club (俱乐部) at Mrs. Green’s house at lunch time today, and I want to go to it. I’ll have you some food for your lunch. Is that all right?”“Oh, yes,” her husband answered, “that’s quite all right. What are you going to leave for my lunch?” “This tin (罐头) of fish,” Mrs. Peter said. “And there are some cold, boiled potatoes (土豆) and some beans (豆子) here, too.”“Good” Then Mrs Peter went to her meeting. All the ladies had lunch at Mrs Green’s, and at three o已知四棱锥P-ABCD的底面ABCD是直角梯形，AD∥BC，∠ABC=90°，PA⊥平面ABCD，AB=BC=2AD，若平面PCD与平面PAB所成二面角的余弦值为63，求PAAD的值．