题目

Our risk of cancer rises rapidly as we age. So it makes sense that the elderly should be routinely screened for new tumors (肿瘤) or doesn’t it? While such tracking of cancer is a good thing in general, researchers are increasingly questioning whether all of this testing is necessary for the elderly. With the percentage of people over age 65 expected to nearly double by 2050, it s important to weigh the health benefits of screening against the risks and costs of routine testing. In many cases, screening can lead to additional examinations and operations to remove cancer, which can已知α∈(0，π)，且sinα＋cosα＝，则cos2α的值为           .