题目
12.已知 (A)=P(B)=dfrac (1)(3) . (A|B)=dfrac (1)(4) ,则 (overline (A)|overline (B))= __ ←
题目解答
答案
：由条件概率公式：$P(A|B)=\dfrac {P(AB)}{P(B)}$
得：$P(AB)=P(A|B)\cdot P(B)=\dfrac {1}{12}$
$P(\overline {A}\overline {B})=1-P(A+B)=1-[P(A)+P(B)-P(AB)]=\dfrac {7}{12}$
$P(\overline {A}|\overline {B})=\dfrac {P(\overline {A}\overline {B})}{P(\overline {B})}=\dfrac {7}{8}$
故答案为：$\dfrac {7}{8}$
$\dfrac {7}{8}$
得：$P(AB)=P(A|B)\cdot P(B)=\dfrac {1}{12}$
$P(\overline {A}\overline {B})=1-P(A+B)=1-[P(A)+P(B)-P(AB)]=\dfrac {7}{12}$
$P(\overline {A}|\overline {B})=\dfrac {P(\overline {A}\overline {B})}{P(\overline {B})}=\dfrac {7}{8}$
故答案为：$\dfrac {7}{8}$
$\dfrac {7}{8}$