回复：依次任取7个小于等于10的正整数，求其和为20的概率？

Hi,Brother,
If you want to know the exact answer you have to use computer.Twft is right,it is from generating function.
007's answer is reasonable.I do it by subnormal distribution.Mean=5.5*7=38.5.Sigma=(38.5-7)/4=7.875,z1=19.5/7.875=-2.476,z2=(20.5-38.6)/7.875=-2.286.P(E)=T(2.476)-T(2.286)=.9933-.9888=.0045.

所有跟帖： 

• I'll give the answer in the next couple days. -皆兄弟也- ♂ (54 bytes) () 06/07/2010 postreply 18:45:32

• 0.0026355 is the exact answer,if I do well. -jinjing- ♀ (221 bytes) () 06/08/2010 postreply 14:36:55

• maybe, you are right. My answer is 0.0026544, but not confident -皆兄弟也- ♂ (0 bytes) () 06/09/2010 postreply 00:20:27