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JEE Advanced 2007 · Probability (Class 12)

Let EcE^{c} denote the complement of an event EE. Let EE, FF, GG be pairwise independent events with P(G)>0P(G) > 0 and P(E∩F∩G)=0P(E \cap F \cap G) = 0. Then P(Ec∩Fc∣G)P(E^{c} \cap F^{c} \mid G) equals

Answer: C. P(Ec)−P(F)P(E^{c}) - P(F)

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Written solution

Ec=complement of EE^c = \text{complement of } E

E, F, G pairwise independent,P(G)>0E,\ F,\ G \text{ pairwise independent}, \quad P(G) > 0

P(E∩F∩G)=0P(E \cap F \cap G) = 0

To find: P(Ec∩Fc∣G)\text{To find: } P(E^c \cap F^c \mid G)

P(A∣G)=P(A∩G)P(G)P(A \mid G) = \dfrac{P(A \cap G)}{P(G)}

Take A=Ec∩Fc\text{Take } A = E^c \cap F^c

N=P(Ec∩Fc∩G)N = P(E^c \cap F^c \cap G)

Ec∩Fc∩G=G∖[(E∩G)∪(F∩G)]E^c \cap F^c \cap G = G \setminus \big[(E \cap G) \cup (F \cap G)\big]

N=P(G)−P(E∩G)−P(F∩G)+P(E∩F∩G)N = P(G) - P(E \cap G) - P(F \cap G) + P(E \cap F \cap G)

P(E∩F∩G)=0⇒N=P(G)−P(E∩G)−P(F∩G)P(E \cap F \cap G) = 0 \Rightarrow N = P(G) - P(E \cap G) - P(F \cap G)

P(E∩G)=P(E)P(G),P(F∩G)=P(F)P(G)P(E \cap G) = P(E)P(G), \quad P(F \cap G) = P(F)P(G)

N=P(G)−P(E)P(G)−P(F)P(G)=P(G) [1−P(E)−P(F)]N = P(G) - P(E)P(G) - P(F)P(G) = P(G)\,[1 - P(E) - P(F)]

Within G, E and F never occur together\text{Within } G,\ E \text{ and } F \text{ never occur together}

P(Ec∩Fc∣G)=NP(G)=1−P(E)−P(F)P(E^c \cap F^c \mid G) = \dfrac{N}{P(G)} = 1 - P(E) - P(F)

1−P(E)−P(F)=(1−P(E))−P(F)=P(Ec)−P(F)1 - P(E) - P(F) = (1 - P(E)) - P(F) = P(E^c) - P(F)

⇒option (3)\Rightarrow \text{option } (3)

P(Ec∩Fc∣G)=P(Ec)−P(F)\boxed{P(E^c \cap F^c \mid G) = P(E^c) - P(F)}

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