概率论与醒统计 A={(1,2),(1,3),(1,4),(2,1),(2,3),(2,4) (3,1),(3,2),(3,4)}, AB={(1,2),(1,3),(2,1),(2,3),(3,1),(3,2)}, 由条件概率的公式得 P(BA=P(AB) P(4) 6/122 9123(3,1), (3,2), (3,4)}, A  {(1,2), (1,3), (1,4), (2,1), (2,3), (2,4), AB  {(1,2), (1,3), (2,1), (2,3), (3,1), (3,2)}, 由条件概率的公式得 ( ) ( ) ( ) P A P AB P B A  9 12 6 12  . 3 2 