题目

 已知全集U={1,2,3,4,5,6,7},集合A={1,3,5,6},则∁UA等于() (A){1,3,5,6}    (B){2,3,7} (C){2,4,7}      (D){2,5,7} 答案：C解析:由题意知∁UA={2,4,7}.已知x=3-23+2，y=3+23-2，求3x2-5xy+3y2的值．