两棵
在线等急啊。。。!感激不尽!!!各栽了两棵
两棵树任选一个的情况有两种所以改了为
2*(2/3)*(1-(2/3))*2*(1/2)*(1/2)=2/9;
都不成活的概率:(1/3)*(1/3)*(1/4)=1/36
成活一棵树的概率:2*(2/3)*(1/3)*(1/4)+2*(1/2)*(1/2)*(1/3)*(1/3)=1/6
成活两颗树的概率:1*(2/3)*(2/3)*(1/4)+1*(1/4)*(1/3)*(1/3)+2*(2/3)*(1/3)*2*(1/2)*(1/2)=13/36
成活三棵树的概率:2*(2/3)*(1/3)*(1/4)+2*(1/2)*(1/2)*(4/9)=1/3
都成活的概率:(2/3)*(2/3)*(1/4)=1/9
E=7/3