If A and B are independent events, then which of the following is not true?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A and B are mutually exclusive **
Given: A and B are independent events.
For independent events: $P(A \cap B) = P(A)P(B)$
Checking each statement:
1. $A'$ and $B'$:
$P(A' \cap B') = 1 - P(A \cup B) = 1 - [P(A) + P(B) - P(A)P(B)] = (1 - P(A))(1 - P(B)) = P(A')P(B')$
⇒ Independent ✔
2. $A$ and $B'$:
$P(A \cap B') = P(A) - P(A \cap B) = P(A) - P(A)P(B) = P(A)[1 - P(B)] = P(A)P(B')$
⇒ Independent ✔
3. $A$ and $B$:
Given independent ✔
4. $A$ and $B$ are mutually exclusive:
For mutually exclusive events, $P(A \cap B) = 0$, which contradicts $P(A)P(B) ≠ 0$ (if both have nonzero probability).
⇒ Not true ❌
Answer: A and B are mutually exclusive.