甲乙两个袋子中，各放有大小和形状、个数相同的小球若干．每个袋子中标号为0的小球为1个，标号为1的2个，标号为2的n个．从一个袋子中 任取两个球，取到的标号都是2的概率是
（1）从甲袋中任取两个球，标号分别是1和2的取法有多少种？ （2）从甲袋中任取两个球，已知其中一个的标号是1的条件下，求另一个标号也是1的概率； （3）从两个袋子中各取一个小球，用ξ表示这两个小球的标号之和，求ξ的分布列和E（ξ）． |
（1）由题意得：
=C 2n C 2n+3
=n(n-1) (n+3)(n+2)
，解得n=2；1 10
记“从甲袋中任取两个球，标号分别是1和2”为事件M，则M的总数是C21=2．
所以从甲袋中任取两个球，标号分别是1和2的取法有2种．
（2）记“一个标号是1”为事件A，“另一个标号也是1”为事件B，
所以P(B|A)=
=P(AB) P(A)
=C 22
-C 25 C 13 1 7
（3）随机变量ξ的分布列为
| ξ | 0 | 1 | 2 | 3 | 4 | ||||||||||
| P |
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| A little girl thought she was not as beautiful as other girls and nobody liked her. So she was always 1 and didn't like to talk to others. However, one day, her mother 2 her a beautiful hair clip (发夹). When she wore it, she looked much more beautiful than 3 . She decided to wear it to school. On her way to school she found that everyone who saw her smiled 4 her. Most of her classmates said 5 to her, but this never happened before. She thought that the beautiful hair clip 6 her happiness. She was so happy about all of the wonderful things. All her classmates wanted to know what happened to her, 7 she didn't want to tell them about her beautiful hair clip. When she went back to her 8 after school, her mother asked her, "Did you know you dropped your hair clip? I 9 it by the door this morning." She understood that she didn't wear the hair clip to school at all. It is not important what we wear or 10 we look. The most important is how we think about ourselves. | |||
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