题目内容

若随机变量X的可能取值为1与a，且P{X=1}=0.4，E(X)=0.2，则a=______．

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The factory operated ( )until the order was filled.

A. at times
B. to the minute
C. day by day
D. around the clock

( )is the study of the characteristics of language varieties, the characteristics of their functions and the characteristics of their speakers as these three constantly interact and change within a speech community.

A. Psycholinguistics
B. Sociolinguistics
C. Anthropological linguistics
D. Computational linguistics

采用无差异目标市场策略的主要优点在于成本低、操作简单，所有客户接受的产品和服务是不同的。

A. 对
B. 错

若E(X)、E(Y)都存在，则下面命题中错误的是______

A. E(X+Y)=E(X)+E(Y)
B. E(X-Y)=E(X)-E(Y)
C. E(6X)=6E(X)
D. E(XY)=E(X)E(Y)