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Let A,B and C be three events, which are...

Let A,B and C be three events, which are pair-wise independent and `bar(E )` denotes the complement of an event E. If `P (A nn B nn C) = 0 and PC) gt 0`, then `P[(bar(A) nn bar(B)) |C]` is equal to :

A

`P(A) + P (bar(B))`

B

`P(bar(A)) - P (bar(B))`

C

`P(bar(A)) P (B)`

D

`P(bar(A)) + P (bar(B))`

Text Solution

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The correct Answer is:
To solve the problem, we need to find \( P(\bar{A} \cap \bar{B} | C) \) given that events \( A, B, \) and \( C \) are pairwise independent, \( P(A \cap B \cap C) = 0 \), and \( P(C) > 0 \). ### Step-by-Step Solution: 1. **Understanding the Conditional Probability**: The conditional probability can be expressed using the formula: \[ P(\bar{A} \cap \bar{B} | C) = \frac{P(\bar{A} \cap \bar{B} \cap C)}{P(C)} \] 2. **Using Independence**: Since \( A, B, \) and \( C \) are pairwise independent, we can express the joint probability of the complements: \[ P(\bar{A} \cap \bar{B} \cap C) = P(\bar{A}) \cdot P(\bar{B}) \cdot P(C) \] 3. **Finding the Complements**: The probabilities of the complements can be expressed as: \[ P(\bar{A}) = 1 - P(A) \quad \text{and} \quad P(\bar{B}) = 1 - P(B) \] 4. **Substituting in the Formula**: Substitute these into the expression for \( P(\bar{A} \cap \bar{B} \cap C) \): \[ P(\bar{A} \cap \bar{B} \cap C) = (1 - P(A))(1 - P(B)) P(C) \] 5. **Substituting Back into the Conditional Probability**: Now substitute this back into the conditional probability formula: \[ P(\bar{A} \cap \bar{B} | C) = \frac{(1 - P(A))(1 - P(B)) P(C)}{P(C)} \] The \( P(C) \) cancels out: \[ P(\bar{A} \cap \bar{B} | C) = (1 - P(A))(1 - P(B)) \] 6. **Using the Given Information**: We know that \( P(A \cap B \cap C) = 0 \). Since \( A, B, \) and \( C \) are independent, this implies: \[ P(A) \cdot P(B) \cdot P(C) = 0 \] Given \( P(C) > 0 \), it follows that: \[ P(A) \cdot P(B) = 0 \] Therefore, at least one of \( P(A) \) or \( P(B) \) must be zero. 7. **Conclusion**: If either \( P(A) = 0 \) or \( P(B) = 0 \), then: \[ P(\bar{A}) = 1 \quad \text{or} \quad P(\bar{B}) = 1 \] Thus, we find: \[ P(\bar{A} \cap \bar{B} | C) = (1 - P(A))(1 - P(B)) = 1 \] ### Final Answer: \[ P(\bar{A} \cap \bar{B} | C) = 1 \]
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