条件概率 21.2.20 ∪4B|=∑P(4|B) 证∵d∪丿 oO ∪4)∩B=∪(4∩B) i=1 且(4∩B)∩(4nB)cA∩A=i≠j P(CAnB PU( ∪A1B P(B) P(B) P(A∩B) ∑ 1 P(B) ∑P(4|B) <u>电子科技大学21.2.20 电子科技大学 条件概率 ( ). 1 1   =  = =        i i i P  Ai B P A B 证 ( ) ( ) 1 1 A B A B i i i   i     =  = = A B A B A A i j 且 ( i  ）( j  ） i  j = ，  ( ) ( ( ) ( ) 1 1 1 P B P A B P B P A B P A B i i i i i i         =         =          =  =  =      ( ). ( ) ( ) 1 1    =  = = = i i i i P A B P B P A  B