题目

     Some people surveyed tasted the sweet, salty, and crunchy together with other tastes of food as they listen to different noises. It shows that louder noises reduced the reported sweetness or saltiness of food while increasing the measure of crunchiness(松脆). It may go some way to explain why airline food is widely complained about by both the workers and passengers. Andy Woods, a researcher, said, “It’s a general opinion that plane foods aren’t fantastic, but I’m sure airlines do their best, so we wondered if there were other reasons. One thought was that perhaps the如图，一个半径为r的⊙O与矩形ABCD的两边AB、BC都相切，BC=4．若将矩形的边AD沿AE对折后和⊙O相切于点D′，折痕AE的长为5，则半径r的值为________．