p{x=1,y=1}=p{x=1）*(y=1}
=c(3,1)*(1/2)^1*(1/2)^(3-1)*c(1,1)*(1/3)^1*(1/3)^(1-1)
=1/8但是答案是1/6p{x=1,y=1}=p{x=1）*(y=1}
=c(1,1)*(1/2)^1*(1/2)^(1-1)*c(1,1)*(1/3)^1*(1/3)^(1-1)
=1/6