题目

从某企业生产的某种产品中抽取500件，测量这些产品的一项质量指标值，由测量结果得如下图频率分布直方图： （I）求这500件产品质量指标值的样本平均值和样本方差（同一组的数据用该组区间的中点值作代表）； （II）由直方图可以认为，这种产品的质量指标服从正态分布，其中近似为样本平均数，近似为样本方差. （i）利用该正态分布，求； （ii）某用户从该企业购买了100件这种产品，记表示这100件产品中质量指标值位于区间的产品件数.利用语法填空（用所给单词的适当形式填空，未提供单词的限填一词） It was reported about ten years ago that mobile phones would be the cigarettes of the 1. (twenty-one) century. That is TRUE! It 2. (seem) that the use of mobile phones for some people is almost as addictive（上瘾的） 3. cigarettes. Many users keep them 4. (work) for hours a day. Many people use them not only 5. (talk) with people, send and receive messages, but also play games, take photos or even get on the Internet to read news. They say they can't live 6. them. But this is not a little thing. In fact too much use of mobile phones may cause 7.（ill）, and many people have got headache by 8. (use) mobile phones too much. Now in other 9. (country), mobile phones carry a health warning on the back just like 10. cigarette packets do.