已知随机变量 ξi 满足 P(ξi=1)=pi ， P(ξi=0)=1−pi ， i=1,2 .若 12<p1<p2<1 ，则

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已知随机变量 ξi 满足 P(ξi=1)=pi ， P(ξi=0)=1−pi ， i=1,2 .若

1

2

<p1<p2<1 ，则（）

A： $E (ξ_{1}) < E (ξ_{2})$ ， $D (ξ_{1}) < D (ξ_{2})$

B： $E (ξ_{1}) < E (ξ_{2})$ ， $D (ξ_{1}) > D (ξ_{2})$

C： $E (ξ_{1}) > E (ξ_{2})$ ， $D (ξ_{1}) < D (ξ_{2})$

D： $E (ξ_{1}) > E (ξ_{2})$ ， $D (ξ_{1}) > D (ξ_{2})$

上一题下一题 0.0难度选择题更新时间:2013-03-04 01:17:55

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B

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Many students in China are learning English. Some of these students are small children Others are teenagers(青少年). Many are adults(成年人). Some learn at school, others study by themselves. A few learn English language over the radio, on television or in films. One must work hard to learn another language.
Why do all these people want to learn English? It is difficult to answer that question. Many boys and girls learn English at school because it is one of their subjects. They study their own(自己的) language, Maths and English. Some people learn English because it is useful(有用的) for their work. Many people often learn English for their further studies, because at college or university(大学) some of their books are in English. Other people learn English because they want to read newspapers and magazines in English.

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