The events , and are such that , and . It is also known that and .
- (a)
Find exactly the maximum and minimum possible values of
(i) ,
- (ii)
- (b)
It is given further that and are independent, find the value of
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Worked solution
6(a)
1. Determine $P(A \cap B \cap C)$ using the given subset condition.
Since , the event must be a subset of ().
Therefore, the intersection of and is entirely contained within , meaning .
2. Express $P(A \cup C)$ in terms of $P(A \cap B^c \cap C)$.
We partition into two mutually exclusive events based on :
Using the addition rule for probabilities:
3. Apply probability bounds to find the maximum and minimum values.
Since any probability must not exceed , we have :
Also, since , its probability is bounded by :
Thus, the maximum possible value is and the minimum possible value is .
6(ii)
1. Determine the required range of values for $P(A \cap B^c \cap C)$ from the previously given probabilities.
Let .
Since , the event is a subset of , meaning .
Therefore, .
Using the addition rule, .
Since any probability cannot exceed , .
Additionally, since , we have .
Thus, .
2. Express the required conditional probability in terms of $x$.
By the definition of conditional probability,
Since is a subset of , the intersection is simply , making the numerator .
From Step 1, we established that .
Thus, the conditional probability expression simplifies to .
3. Find the maximum and minimum of the conditional probability using the feasible range of $x$.
We define the function over the domain .
We can rewrite this algebraically:
As increases, the denominator decreases, causing the fraction to increase. Hence, the function is strictly increasing over the interval .
The minimum possible value occurs at :
The maximum possible value occurs at :
Maximum is , minimum is .
6(b)
1. Calculate the probability of the intersection of independent events $A$ and $C$.
Since and are independent,
2. Express $P(A \cap C)$ in terms of its disjoint subsets and solve for the required probability.
Given , we have .
This implies that .
Since ,
Now, decomposing into disjoint subsets involving and :
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