题目

Last weekend, our school organized some volunteers to help ____ the traffic at the main crossroads. ___, I was one of them.Early in the ____, forty of us volunteers got together at the school gate. Before we left, our teacher told us some ____ do’s and don’ts for traffic police. Then we were ____ hats, fluorescent vests (荧光马甲) and red flags. It was really ____ to be fully equipped for the sacred (神圣的) mission!Eventually, our group ____ the appointed (指定的) crossroads and one of the ____ policewomen welcomed us warmly. She ____ us into four groups, each of us standing at 已知x+y=4，xy=3，求下列代数式的值：（1）x2+y2；（2）x2﹣y2．