问题 选择题
已知A箱内有红球1个和白球(n+1)个,B箱内有白球(n-1)个(n∈N,且n≥2),现随意从A箱中取出3个球放入B箱,将B箱中的球充分搅匀后,再从中随意取出3个球放入A箱,则红球由A箱移到B箱,再返回到A箱的概率等于(  )
A.
2
n+1
B.
3
n+2
C.
9
(n+2)2
D.
1
(n+1)2
答案

由题可知:第一步:A箱内有红球1个和白球(n+1)个共n+2个球.从A箱中取出3个球放入B箱,红球从A箱移到B箱概率为:

c11
×
c2n+1
c3n+2
.此时,B箱内共有n+2个球:一个红球,n+1个白球;

第二步:将B箱中的球充分搅匀后,再从中随意取出3个球放入A箱,则红球放入A箱的概率为:

c11
×
c2n+1
c3n+2

∴红球由A箱移到B箱,再返回到A箱的概率等于

c11
×
c2n+1
c3n+2
×
c11
×
c2n+1
c3n+2
=
9
(n+2)2
.故答案:选C.

问答题
填空题

Holidays are special times of (1) from work and other routines. Holidays are often times for celebration, (2) , eating, drinking, travel, and family (3) . In most cultures the scheduling of holidays originally was related to the (4) , the (5) cycle, and (6) . Christmas, December 25, celebrates the (7) of Jesus, but it is not actually known whether Jesus was (8) in the wintertime Over the years Christmas has come to symbolize goodwill and (9) for both Christians and non-Christians, and the (10) of Christmas threatens to replace generosity with greed. Other important holidays in America are Valentine’s Day, St. Patrick’s Day, April Fools’ Day, and (11) . On Valentine’s Day, people give cards, chocolates, flowers, and kisses to their (12) and sweethearts. During Easter Week in late March or early April, Christians remember the death and (13) of Jesus. Although not actually a holiday and has no religious (14) , April Fools’ Day, celebrated on April 1, is a day when people play embarrassing (15) on their fiends and (16) and even on their teachers. In (17) , Labor Day is celebrated on May 1, whereas in (18) and the United States, labor and laborers are honored by a (19) holiday on the first (20) in September.

Holidays are special times of (1) from work and other routines. Holidays are often times for celebration, (2) , eating, drinking, travel, and family (3) . In most cultures the scheduling of holidays originally was related to the (4) , the (5) cycle, and (6) . Christmas, December 25, celebrates the (7) of Jesus, but it is not actually known whether Jesus was (8) in the wintertime Over the years Christmas has come to symbolize goodwill and (9) for both Christians and non-Christians, and the (10) of Christmas threatens to replace generosity with greed. Other important holidays in America are Valentine’s Day, St. Patrick’s Day, April Fools’ Day, and (11) . On Valentine’s Day, people give cards, chocolates, flowers, and kisses to their (12) and sweethearts. During Easter Week in late March or early April, Christians remember the death and (13) of Jesus. Although not actually a holiday and has no religious (14) , April Fools’ Day, celebrated on April 1, is a day when people play embarrassing (15) on their fiends and (16) and even on their teachers. In (17) , Labor Day is celebrated on May 1, whereas in (18) and the United States, labor and laborers are honored by a (19) holiday on the first (20) in September.