For three events A, B and C, P (Exactly one of A or B occurs) = P (Exactly one of B or C occurs) = P(Exactly one of C or A occurs) = and P (All the three events occur simultaneously) =
Then the probability that at least one of the events occurs, is
P (exactly one of A & B occur) = P(A) + P(B) – …….(1)
P (exactly one of B & C occur) = P(B) + P(C) – …….(2)
P (exactly one of C & A occur) = P(C) + P(A) – …….(3)
Adding (1), (2) & (3) —————————————
2(P(A) + P(B) + P(C) – P(A B) – P(B C) – P(C A)) =
⇒ P(A) + P(B) + P(C) – P(A B) – P(B C) – P(C A) =
P(All three occurs simultaneously) = P(A B C) =
P (atleast one of events occur) = P(A B C) = .
Let's analyze the problem step by step. We are given three events A, B, and C with the following probabilities:
We need to find P(at least one of A, B, or C occurs).
Step 1: Express "Exactly one of A or B occurs" in terms of set operations.
Exactly one of A or B occurs means A occurs without B, or B occurs without A. This is equivalent to (A ∩ B') ∪ (A' ∩ B). The probability is:
P(A) - P(A ∩ B) + P(B) - P(A ∩ B) = P(A) + P(B) - 2P(A ∩ B)
So, P(A) + P(B) - 2P(A ∩ B) = ...(1)
Similarly, for B and C: P(B) + P(C) - 2P(B ∩ C) = ...(2)
For C and A: P(C) + P(A) - 2P(C ∩ A) = ...(3)
Step 2: Add equations (1), (2), and (3):
2[P(A) + P(B) + P(C)] - 2[P(A∩B) + P(B∩C) + P(C∩A)] =
Divide both sides by 2:
P(A) + P(B) + P(C) - [P(A∩B) + P(B∩C) + P(C∩A)] = ...(4)
Step 3: Use the inclusion-exclusion principle for three events.
P(at least one occurs) = P(A ∪ B ∪ C) = P(A) + P(B) + P(C) - P(A∩B) - P(B∩C) - P(C∩A) + P(A∩B∩C)
From equation (4), we have P(A) + P(B) + P(C) - [P(A∩B) + P(B∩C) + P(C∩A)] =
And P(A∩B∩C) =
So, P(A ∪ B ∪ C) = + = + =
Final Answer:
Inclusion-Exclusion Principle: For any two events A and B, P(A ∪ B) = P(A) + P(B) - P(A ∩ B). For three events A, B, C, P(A ∪ B ∪ C) = P(A) + P(B) + P(C) - P(A ∩ B) - P(B ∩ C) - P(C ∩ A) + P(A ∩ B ∩ C). This principle helps in finding the probability of the union of events.
Probability of Exactly One Event: The probability that exactly one of two events A or B occurs is given by P(A) + P(B) - 2P(A ∩ B). This is derived from the fact that it includes outcomes in A but not in B, and in B but not in A.