Analyzing the Setup
We are tasked with finding the probability of the intersection of the complements of two events, E and F, given that event G has occurred. The expression to evaluate is P(Ec∩Fc∣G).
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 P(E∪F∣G), we utilize the Inclusion-Exclusion Principle:
P(E∪F∣G)=P(E∣G)+P(F∣G)−P(E∩F∣G)
The Power of Pairwise Independence
We are given that E,F, and G are pairwise independent. This implies that the occurrence of G provides no information about E or F. Mathematically, this simplifies our terms:
Substituting these into our previous expansion, the expression becomes:
Final Calculation
Finally, we evaluate the term P(E∩F∣G). By the definition of conditional probability:
Since the problem states that P(E∩F∩G)=0 and P(G)>0, this term vanishes. We are left with:
Recognizing that 1−P(E)=P(Ec), the final, elegant result is:
P(Ec)−P(F)