in Probability edited by
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8 votes
8 votes

Which of the following statements is NOT true?

  1. If $A \subset B$, then $P(A) \leq P(B)$.
  2. If $P(B)>0$, then $P(A \mid B) \geq P(A)$.
  3. $P(A \cap B) \geq P(A)+P(B)-1$.
  4. $P\left(A \cap B^c\right)=P(A \cup B)-P(B)$.
in Probability edited by
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2 Answers

11 votes
11 votes
A counter example: if we have two events $A, B$ such that $P(B)>0$ and $P(A)>0$, but $A \cap B=\emptyset$, then $P(A \mid B)=0,$ but $P(A)>P(A \mid B)$. It's easy to come up with examples like this: for example, take any sample space with event $A$ such that $P(A)>0,$ and $P\left(A^c>0\right)$, it follows that $P\left(A \mid A^c\right)=0,$ but $P(A)>0.$
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6 votes
6 votes

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4 Comments

shouldn’t P(B) be 4/8, though it will reduce to same value but if we count the elements out of total then..
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yep.
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How option C is correct?
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Rearrange the terms 

Bring the 1 from RHS to LHS.Now even if P(A ∩ B) is 0,the LHS would be still greater than equal to P(A)+P(B), no matter what the probabilities of both the events might be.

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