题目

设全集U = {1,2,3,4,5,6,7,8},集合S = {1,3,5},T = {3,6}，则等于 A．Φ      B．{2,4,7,8}  C．{1,3,5,6}  D．{2,4,6,8} 答案：B已知x2=4，则方程的根为 ．