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南京二模19题 条件期望

  • 2026-05-12 14:42:06
南京二模19题 条件期望

第(2)问

分两种情况：

  1. 最后一轮取出2个红球

  2. 第  轮和第  轮分别取出1个红球

故

第(3)问

 可取  和 ，记摸球  次结束且乙分别摸到1个和2个红球的概率分别为 ，则 

故 

只需证 

类似第(2)问，分两种情况计算 :

于是

得证.

总结

本题属于条件期望问题， 与  都是条件概率.

难点在于计算.

质量：★★☆☆☆难度：★★★☆☆

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  1. CONNECT:[ UseTime:0.001073s ] mysql:host=127.0.0.1;port=3306;dbname=b460;charset=utf8mb4
  2. SHOW FULL COLUMNS FROM `fenlei` [ RunTime:0.001563s ]
  3. SELECT * FROM `fenlei` WHERE `fid` = 0 [ RunTime:0.000722s ]
  4. SELECT * FROM `fenlei` WHERE `fid` = 63 [ RunTime:0.000767s ]
  5. SHOW FULL COLUMNS FROM `set` [ RunTime:0.001424s ]
  6. SELECT * FROM `set` [ RunTime:0.026221s ]
  7. SHOW FULL COLUMNS FROM `article` [ RunTime:0.001568s ]
  8. SELECT * FROM `article` WHERE `id` = 554848 LIMIT 1 [ RunTime:0.000983s ]
  9. UPDATE `article` SET `lasttime` = 1778598398 WHERE `id` = 554848 [ RunTime:0.013903s ]
  10. SELECT * FROM `fenlei` WHERE `id` = 65 LIMIT 1 [ RunTime:0.002203s ]
  11. SELECT * FROM `article` WHERE `id` < 554848 ORDER BY `id` DESC LIMIT 1 [ RunTime:0.001230s ]
  12. SELECT * FROM `article` WHERE `id` > 554848 ORDER BY `id` ASC LIMIT 1 [ RunTime:0.001086s ]
  13. SELECT * FROM `article` WHERE `id` < 554848 ORDER BY `id` DESC LIMIT 10 [ RunTime:0.001900s ]
  14. SELECT * FROM `article` WHERE `id` < 554848 ORDER BY `id` DESC LIMIT 10,10 [ RunTime:0.015559s ]
  15. SELECT * FROM `article` WHERE `id` < 554848 ORDER BY `id` DESC LIMIT 20,10 [ RunTime:0.007918s ]
0.253764s