已知事件 、 和 满足 、 且 。 和 是独立事件。 和 是互斥事件。
- (a)
写出 和 的值。
- (b)
已知 。
求 的值。
- (c)
求 的最大可能值和最小可能值。
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完整解法
8(a)
1. State the value of $\mathrm{P}(A | B)$ using the property of independent events
Since and are independent events, the occurrence of does not affect the probability of .
2. State the value of $\mathrm{P}(A | C)$ using the property of mutually exclusive events
Since and are mutually exclusive events, they cannot occur at the same time, meaning .
,
8(b)
1. Calculate $\mathrm{P}(A \cup B)$ using the independence of $A$ and $B$.
Since and are independent, .
2. Express $\mathrm{P}(A' \cap C')$ in terms of $c$ using the fact that $A$ and $C$ are mutually exclusive.
Since and are mutually exclusive, .
Using De Morgan's laws, .
3. Equate the two probabilities and solve for $c$.
Equating to :
8(c)
1. Express $\mathrm{P}(A' \cap B' \cap C')$ in terms of the unknown probability $x = \mathrm{P}(B \cap C)$.
From earlier parts, and .
Let . Since and are mutually exclusive, and do not overlap, meaning and .
By considering the separate, non-overlapping regions in a Venn diagram:
The probability of the region entirely outside all three sets is:
2. Determine the valid range of values for $x = \mathrm{P}(B \cap C)$.
For the probabilities of the individual regions to be valid, they must be non-negative:
Combining these conditions, the most restrictive bounds give the valid range for as:
3. Calculate the minimum and maximum possible values of $\mathrm{P}(A' \cap B' \cap C')$.
Substitute the minimum and maximum valid values of into the expression for :
Maximum , Minimum
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