l 34 210210210 0 2102102100 33030 210210210 0 000 210210 等品不超过2个,二等品至少3个的概率为: P(0≤X≤2,3≤Y≤4 =P(X=0.,y=3)+P(X=0,y=4)+P(X=1,Y=3) +P(X=1,X=4)+P(X=2,=3)+P(X=2,y 20530 ++0+0+0 210210210 210Y X 012 3 4 0 00 10 210 20 210 5 210 1 0 15 60 30 210 210 210 0 2 3 210 30 210 30 210 0 0 210 3 2 210 5 210 00 0 一等品不超过 2 个，二等品至少 3 个的概率为： PX Y ( ,) 0 23 4 ≤ ≤ ≤≤ = P(X = 0 Y = 3) + P(X = 0 Y = 4) + P(X = 1 Y = 3) + = =+ = =+ = = P X Y P X Y P X Y PX Y PX Y PX Y ( , ) ( , ) ( , ) ( , )( , )( , ) 0 3 0 4 1 3 14 23 24 = + + +++ 20 210 5 210 30 210 000 = 55 210