1.P(甲乙成功)=C(6,4)*(C(2,2)*(C(4,2))/C(6,4)*C(6,4)=6/15=2/5;
2.P(甲乙成功)=P(乙丙成功)=P(甲丙成功)=2/5
P (x=0)=(3/5)^3=27/125,P(x=1)=3*2/5*(3/5)^2=54/125,P(x=2)=3*(2/5)^2*3/5=36/125,
P(x=3)=(2/5)^3=8/125,Ex=3*2/5=1.2
1.P(甲乙成功)=C(6,4)*(C(2,2)*(C(4,2))/C(6,4)*C(6,4)=6/15=2/5;
2.P(甲乙成功)=P(乙丙成功)=P(甲丙成功)=2/5
P (x=0)=(3/5)^3=27/125,P(x=1)=3*2/5*(3/5)^2=54/125,P(x=2)=3*(2/5)^2*3/5=36/125,
P(x=3)=(2/5)^3=8/125,Ex=3*2/5=1.2