题目

阅读理解 For an increasing number of students at American universities, Old is suddenly in. The reason is obvious: the graying of America means jobs, coupled with the aging of the baby-boom generation, a longer life span means that the nation's elderly population is bound to expand significantly over the next 50 years. By 2050, 25 percent of all Americans will be older than 65, up from 14 percent in 1995.The change poses profound questions for government and society, of course. But it also creates career opportunities in medicine and health professions and in law and business as well. “I如图，在直三棱柱ABC-A1B1C1中，底面ABC是等腰直角三角形，且，侧棱，点D是A1B1的中点，则异面直线B1C与AD所成的角的余弦值是    ．