图解max{X,Y}或min{X,Y}并求相关概率
对max{X,Y}或min{X,Y}进行分解再求解
P ( m a x { X , Y } ≥ c ) = P [ ( X ≥ c ) ∪ ( Y ≥ c ) ] P ( m a x { X , Y } ≤ c ) = P [ ( X ≤ c ) ∩ ( Y ≤ c ) ] P ( m i n { X , Y } ≥ c ) = P [ ( X ≥ c ) ∩ ( Y ≥ c ) ] P ( m i n { X , Y } ≤ c ) = P [ ( X ≤ c ) ∪ ( Y ≤ c ) ] P(max\{X,Y\}\geq c)=P[(X\geq c)\cup(Y\geq c)]\\ P(max\{X,Y\}\leq c)=P[(X\leq c)\cap(Y\leq c)]\\ P(min\{X,Y\}\geq c)=P[(X\geq c)\cap(Y\geq c)]\\ P(min\{X,Y\}\leq c)=P[(X\leq c)\cup(Y\leq c)]\\ P(max{X,Y}≥c)=P[(X≥c)∪(Y≥c)]P(max{X,Y}≤c)=P[(X≤c)∩(Y≤c)]P(min{X,Y}≥c)=P[(X≥c)∩(Y≥c)]P(min{X,Y}≤c)=P[(X≤c)∪(Y≤c)]
先来图解一下上述结论
P ( m a x { X , Y } ≥ c ) = P [ ( X ≥ c ) ∪ ( Y ≥ c ) ] P(max\{X,Y\}\geq c)=P[(X\geq c)\cup(Y\geq c)] P(max{X,Y}≥c)=P[(X≥c)∪(Y≥c)]
P ( m a x { X , Y } ≤ c ) = P [ ( X ≤ c ) ∩ ( Y ≤ c ) ] P(max\{X,Y\}\leq c)=P[(X\leq c)\cap(Y\leq c)] P(max{X,Y}≤c)=P[(X≤c)∩(Y≤c)]
P ( m i n { X , Y } ≥ c ) = P [ ( X ≥ c ) ∩ ( Y ≥ c ) ] P(min\{X,Y\}\geq c)=P[(X\geq c)\cap(Y\geq c)] P(min{X,Y}≥c)=P[(X≥c)∩(Y≥c)]
P ( m i n { X , Y } ≤ c ) = P [ ( X ≤ c ) ∪ ( Y ≤ c ) ] P(min\{X,Y\}\leq c)=P[(X\leq c)\cup(Y\leq c)] P(min{X,Y}≤c)=P[(X≤c)∪(Y≤c)]
要注意区别 m a x ( X , Y ) ≤ c max(X,Y)\leq c max(X,Y)≤c 和 m i n ( X , Y ) ≤ c min(X,Y)\leq c min(X,Y)≤c 的示意图
不知道各位读者注意到了没有,在画 X = c X=c X=c 和 Y = c Y=c Y=c 时左右两个图是有区别的,这是由于上图左侧图像中 Y = X Y=X Y=X的上半部分是 Y Y Y 下半部分是 X X X 所以在画 X = c X=c X=c 时虚线只划到取 X X X 的部分,在画 Y = c Y=c Y=c 时虚线只画到取 Y Y Y 的部分,上图右侧图像同理如此。
例:2006年数学一