题目

每人在一轮投篮练习中最多可投篮4次,现规定一旦命中即停止该轮练习,否则一直试投到4次为止.已知一选手的投篮命中率为0.7,求一轮练习中该选手的实际投篮次数ξ的分布列,并求出ξ的期望Eξ与方差Dξ（保留3位有效数字）. 答案：解:ξ的取值为1,2,3,4.ξ=1,表示第一次即投中,故P（ξ=1）=0.7;ξ=2,表示第一次未投中,第二次投中,故P（ξ=2）=（1－0.7）×0.7=0.21;ξ=3,表示第一、二次未投中,第三次投中,故P（ξ=3）=（1－0.7）2×0.7=0.063;ξ=4,表示前三次均未投中,故P（ξ=4）=（1－0.7）3=0.027.所以ξ的分布列为ξ1234P0.70.210.0630.027Eξ=1×0.7+2×0.21+3×0.063+4× China is a country that is changing very quickly over the past fifty years and_________ you look closely, you can see it_________in front of your eyes. Fifty years ago, everybody travelled around by bike. Today lots of people in China still ride their bikes _________school or work, but many people also take the bus or travel ________underground. There are some people who even drive their own cars. Shops _________ small businesses（商店） but now the centre of China’s cities are full of ________department stores. In these department stores, you can buy ________you want in one trip. You can buy food, toys, bikes and even televisions. Television is ________change that China has been. Many children would say that they cannot imagine life ________, but years ago they had to live without them. The Internet and the computer have only recently become regular items in our lives. Without the Internet, people would not _________to get information as quickly as they can now and the Internet helps people _________friends all over the world. Computers help ________children and adults in their _________lives. Many students are lucky enough ________able to use them to do their homework or ________with their friends and sometimes play games.【1】A.even ifB.ifC.althoughD.so【2】A.be changingB.to changingC.changingD.changes【3】A.fromB.toC.inD.at【4】A.withB.atC.inD.by【5】A.used to beB.used to beingC.used to isD.was used to being【6】A.largeB.largerC.smallD.smaller【7】A.anythingB.somethingC.everythingD.nothing【8】A.othersB.oneC.the othersD.another【9】A.with oneB.without oneC.with itD.without it【10】A.can be able toB.be able toC.able toD.could【11】A.beingB.to have beenC.beD.have been【12】A.bothB.allC.eitherD.neither【13】A.every dayB.everydayC.some dayD.someday【14】A.beB.beingC.to beD.to【15】A.speakB.chattingC.chatD.speaking