2022周报七年级英语38期答案

18.【解析】(1)记这50名员工学习得分的平均数为,则x=×(5×5+15×10+25×15+35×13+45×7)=26.4.………4分(2)用分层抽样可知从[10,20)中选2人,记这2人分别为a1、a2从[2030)中选3人,记这3人分别为b1、b2、b2………6分从a1、a、b、b2、b中再任取2人的情况有:a1a2、a1b1、a1b2、a1b、a2b、a2b2、a2b3、b1b2、b1b3、b2b,共10种;…………8分其中得分在[0,20)和[20,30)中各有1人的情况有a1b、a1bna1b、a2b、a2b2、a2b3,共6种.…10分记事件A为“得分在[10,20)和[20,30)中各有1人”则P(A)=5=……12分

Dear TomHow are you doing? In the last letter, you asked me about our school's online activitiesSNow I'll share something with youLike many other schools, we also use Ding Talk to attend classes and have videoconferences. We even hand in homework and take exams online. Using Ding Talk, ourteachers correct our homework and explain difficult points to us at any time only with a mobilephone. It is really convenient.However, every coin has two sides. Attending online classes, we have to stare at thescreen for a long time, which is bad for our eyes. What's more, the instability of the Internetis also a drawback of the online activitiesI think you can experience it byy yourselfYours sincerelyeLi Hu

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