Sigma Percentile
JEE Advanced 2007
LEVELJEE Main

Animated Solution for Mathematics - Probability: Let denote the complement of an event . Let be pairwise independent events with and . Then equals

Select Answer:

Visualized Solution

  • Given events: are pairwise independent.
  • and .
  • Goal: Find .

  • Using De Morgan's Law: .
  • Therefore, .

  • Property: .
  • Applying this: .

  • Inclusion-Exclusion Principle:
  • .

  • Since are pairwise independent:

  • Given .

  • So, .

  • Substitute back into the expression:

  • Since :
  • Final Answer:

The Sigma Insight: Conditional Probability

Solution Diagram

Analyzing the Setup

We are tasked with finding the probability of the intersection of the complements of two events, and , given that event has occurred. The expression to evaluate is .

The De Morgan's Insight

Our first step is to simplify the expression using De Morgan's Law. This law allows us to rewrite the intersection of complements as the complement of the union:
Consequently, our target probability transforms into:

The Conditional Bridge

Next, we apply the fundamental property of conditional probability, which states that the probability of the complement of an event is one minus the probability of the event itself. This yields:
To expand , we utilize the Inclusion-Exclusion Principle:

The Power of Pairwise Independence

We are given that and are pairwise independent. This implies that the occurrence of provides no information about or . Mathematically, this simplifies our terms:
Substituting these into our previous expansion, the expression becomes:

Final Calculation

Finally, we evaluate the term . By the definition of conditional probability:
Since the problem states that and , this term vanishes. We are left with:
Recognizing that , the final, elegant result is:

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