解:设事件ξ1 = {A同学排在第一位},ξ2 = {B同学排在第二位},ξ3 = {C同学排在第三位},ξ4 = {D同学排在第四位}
换言之,ξ1非 = {A同学不排在第一位},ξ2非 = {B同学不排在第二位},ξ3非 = {C同学不排在第三位},ξ4非 = {D同学不排在第四位}
于是, __ __ __ __
P 【(ξ1) * (ξ2)* (ξ3) * (ξ4)】
= 1 - P【ξ1 + ξ2 + ξ3 + ξ4】
而,P【ξ1 + ξ2 + ξ3 + ξ4】
= P(ξ1) + P(ξ2) + P(ξ3) + P(ξ4) - P(ξ1ξ2 - P(ξ1ξ3) - P(ξ1ξ4) - P(ξ2ξ3) - P(ξ2ξ4) - P(ξ3ξ4) + P(ξ1ξ2ξ3) + P(ξ1ξ2ξ4) + P(ξ1ξ3ξ4) + P(ξ2ξ3ξ4) - P(ξ1ξ2ξ3ξ4)
= (1/4!) *【3! + 3! + 3! + 3! - 2! - 2! - 2! - 2! - 2! - 2! + 1 + 1 + 1 + 1 -1】
= 15/24
__ __ __ __
∴ P 【(ξ1) * (ξ2)* (ξ3) * (ξ4)】 = 1 - 15/24 = 9/24
∴所求的排列个数 = (9/24) * 4! = 9(种)
追答也可以做一个树形图
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/b2de9c82d158ccbf9db636961bd8bc3eb1354110?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/0ff41bd5ad6eddc44e9637573bdbb6fd5266332c?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/d0c8a786c9177f3e638f7fa172cf3bc79e3d56e6?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
追问谢谢了!