已知事件 AA、BB 和 CC 满足 P(A)=0.8P(A)=0.8,P(B)=0.2P(B)=0.2 且 P(C)=0.6P(C)=0.6。此外还已知 P(A∩B)=P(B)P(A\cap B)=P(B) 且 P(B∩C)=0.1P(B\cap C)=0.1。

  1. (a)

    精确求出

    (i) P(A∩Bc∩C)P(A\cap B^{c}\cap C) 的最大可能值和最小可能值,

  2. (ii)

    P(A∩Bc∩C∣A∪C).P(A\cap B^{c}\cap C\mid A\cup C).

  3. (b)

    又已知 AA 与 CC 相互独立,求 P(A∩Bc∩C)P(A\cap B^{c}\cap C) 的值。

先不看答案,让 AI 诊断你卡在哪里?

它不会直接告诉你答案,而是问你卡在哪一步看看 AI 是怎么陪你一步步弄懂这道题的

完整解法

6(a)

1. Determine $P(A \cap B \cap C)$ using the given subset condition.

Since P(A∩B)=P(B)=0.2P(A \cap B) = P(B) = 0.2, the event BB must be a subset of AA (B⊆AB \subseteq A).
Therefore, the intersection of BB and CC is entirely contained within AA, meaning A∩B∩C=B∩CA \cap B \cap C = B \cap C.
P(A∩B∩C)=P(B∩C)=0.1P(A \cap B \cap C) = P(B \cap C) = 0.1

2. Express $P(A \cup C)$ in terms of $P(A \cap B^c \cap C)$.

We partition A∩CA \cap C into two mutually exclusive events based on BB:
P(A∩C)=P(A∩B∩C)+P(A∩Bc∩C)P(A \cap C) = P(A \cap B \cap C) + P(A \cap B^c \cap C)
P(A∩C)=0.1+P(A∩Bc∩C)P(A \cap C) = 0.1 + P(A \cap B^c \cap C)
Using the addition rule for probabilities:
P(A∪C)=P(A)+P(C)−P(A∩C)P(A \cup C) = P(A) + P(C) - P(A \cap C)
P(A∪C)=0.8+0.6−(0.1+P(A∩Bc∩C))P(A \cup C) = 0.8 + 0.6 - (0.1 + P(A \cap B^c \cap C))
P(A∪C)=1.3−P(A∩Bc∩C)P(A \cup C) = 1.3 - P(A \cap B^c \cap C)

3. Apply probability bounds to find the maximum and minimum values.

Since any probability must not exceed 11, we have P(A∪C)≤1P(A \cup C) \le 1:
1.3−P(A∩Bc∩C)≤11.3 - P(A \cap B^c \cap C) \le 1
P(A∩Bc∩C)≥0.3P(A \cap B^c \cap C) \ge 0.3
Also, since A∩C⊆CA \cap C \subseteq C, its probability is bounded by P(C)P(C):
P(A∩C)≤P(C)P(A \cap C) \le P(C)
0.1+P(A∩Bc∩C)≤0.60.1 + P(A \cap B^c \cap C) \le 0.6
P(A∩Bc∩C)≤0.5P(A \cap B^c \cap C) \le 0.5
Thus, the maximum possible value is 0.50.5 and the minimum possible value is 0.30.3.

答案

Maximum =0.5,Minimum =0.3\text{Maximum } = 0.5, \quad \text{Minimum } = 0.3

6(ii)

1. Determine the required range of values for $P(A \cap B^c \cap C)$ from the previously given probabilities.

Let x=P(A∩Bc∩C)x = P(A \cap B^c \cap C).
Since P(A∩B)=P(B)=0.2P(A \cap B) = P(B) = 0.2, the event BB is a subset of AA, meaning A∩B∩C=B∩CA \cap B \cap C = B \cap C.
Therefore, P(A∩C)=P(A∩B∩C)+P(A∩Bc∩C)=0.1+xP(A \cap C) = P(A \cap B \cap C) + P(A \cap B^c \cap C) = 0.1 + x.
Using the addition rule, P(A∪C)=P(A)+P(C)−P(A∩C)=0.8+0.6−(0.1+x)=1.3−xP(A \cup C) = P(A) + P(C) - P(A \cap C) = 0.8 + 0.6 - (0.1 + x) = 1.3 - x.
Since any probability cannot exceed 11, P(A∪C)≤1  ⟹  1.3−x≤1  ⟹  x≥0.3P(A \cup C) \le 1 \implies 1.3 - x \le 1 \implies x \ge 0.3.
Additionally, since A∩Bc∩C⊆Bc∩CA \cap B^c \cap C \subseteq B^c \cap C, we have x≤P(C)−P(B∩C)=0.6−0.1=0.5x \le P(C) - P(B \cap C) = 0.6 - 0.1 = 0.5.
Thus, 0.3≤x≤0.50.3 \le x \le 0.5.

2. Express the required conditional probability in terms of $x$.

By the definition of conditional probability,
P(A∩Bc∩C∣A∪C)=P(A∩Bc∩C∩(A∪C))P(A∪C)P(A \cap B^c \cap C \mid A \cup C) = \frac{P(A \cap B^c \cap C \cap (A \cup C))}{P(A \cup C)}
Since A∩Bc∩CA \cap B^c \cap C is a subset of A∪CA \cup C, the intersection is simply A∩Bc∩CA \cap B^c \cap C, making the numerator P(A∩Bc∩C)=xP(A \cap B^c \cap C) = x.
From Step 1, we established that P(A∪C)=1.3−xP(A \cup C) = 1.3 - x.
Thus, the conditional probability expression simplifies to x1.3−x\frac{x}{1.3 - x}.

3. Find the maximum and minimum of the conditional probability using the feasible range of $x$.

We define the function f(x)=x1.3−x\mathrm{f}(x) = \frac{x}{1.3 - x} over the domain 0.3≤x≤0.50.3 \le x \le 0.5.
We can rewrite this algebraically:
x1.3−x=−(1.3−x)+1.31.3−x=−1+1.31.3−x\frac{x}{1.3 - x} = \frac{-(1.3 - x) + 1.3}{1.3 - x} = -1 + \frac{1.3}{1.3 - x}
As xx increases, the denominator (1.3−x)(1.3 - x) decreases, causing the fraction 1.31.3−x\frac{1.3}{1.3 - x} to increase. Hence, the function is strictly increasing over the interval [0.3,0.5][0.3, 0.5].
The minimum possible value occurs at x=0.3x = 0.3:
Minimum=0.31.3−0.3=0.31=310\text{Minimum} = \frac{0.3}{1.3 - 0.3} = \frac{0.3}{1} = \frac{3}{10}
The maximum possible value occurs at x=0.5x = 0.5:
Maximum=0.51.3−0.5=0.50.8=58\text{Maximum} = \frac{0.5}{1.3 - 0.5} = \frac{0.5}{0.8} = \frac{5}{8}

答案

Maximum is 58\frac{5}{8}, minimum is 310\frac{3}{10}.

6(b)

1. Calculate the probability of the intersection of independent events $A$ and $C$.

Since AA and CC are independent,
P(A∩C)=P(A)P(C)=0.8×0.6=0.48P(A \cap C) = P(A)P(C) = 0.8 \times 0.6 = 0.48

2. Express $P(A \cap C)$ in terms of its disjoint subsets and solve for the required probability.

Given P(A∩B)=P(B)=0.2P(A \cap B) = P(B) = 0.2, we have P(A′∩B)=0P(A' \cap B) = 0.
This implies that P(A′∩B∩C)=0P(A' \cap B \cap C) = 0.
Since P(B∩C)=P(A∩B∩C)+P(A′∩B∩C)P(B \cap C) = P(A \cap B \cap C) + P(A' \cap B \cap C),
P(A∩B∩C)=0.1−0=0.1P(A \cap B \cap C) = 0.1 - 0 = 0.1
Now, decomposing P(A∩C)P(A \cap C) into disjoint subsets involving BB and BcB^c:
P(A∩C)=P(A∩B∩C)+P(A∩Bc∩C)P(A \cap C) = P(A \cap B \cap C) + P(A \cap B^c \cap C)
0.48=0.1+P(A∩Bc∩C)0.48 = 0.1 + P(A \cap B^c \cap C)
P(A∩Bc∩C)=0.48−0.1=0.38P(A \cap B^c \cap C) = 0.48 - 0.1 = 0.38

答案

0.380.38

AI 分析,可能出错

用 AI 批改,圈出写错的那一步,并自动将你的薄弱点存入错题本,形成你的错因库。

同一知识点的其他题

  1. 1已知事件 AA 和 BB 满足 P(B)=13\mathrm{P}(B) = \frac{1}{3},P(A∣B)=25\mathrm{P}(A|B) = \frac{2}{5} 且 P(A∪B)=35\mathrm{P}(A \cup B) = \frac{3}{5}。…
  2. 2凯(Kai)和里奥(Leo)两兄弟是定期训练的双项全能(跑步和骑行)运动员。在每一天中,凯骑行的概率为 34\frac{3}{4},跑步的概率为 35\frac{3}{5},两者都进行的概率为 pp。 平均而言,里奥在一周的 7 天中有…
  3. 3一对同胞 Abbey 和 Betty 还有其他六个朋友一起去参加书展。他们八个人随机排成一队等候一位著名作家的签名。求满足以下条件的概率: Abbey 排在队列第一位且 Betty 排在队列第六位, Abbey 排在队列第一位或 Betty…
  4. 4已知某种特定疾病的感染率约为万分之一(每 10 000 人中约有 1 人感染)。针对该疾病的一项检测准确率为 99.9%,即检测结果与真实情况相符的概率为 99.9%。求在已知某人检测结果呈阳性的条件下,此人确实患有该疾病的概率。 研究人员…
  5. 5对于事件 AA 和 BB,已知 P(A)=0.55\mathrm{P}(A) = 0.55,P(B)=0.65\mathrm{P}(B) = 0.65 且 P(A∪B)=0.93\mathrm{P}(A \cup B) = 0.93。 求 P(A∩B)\mathrm{P}(A \cap B)…