MCQ

Choose the correct answer.

If $\text{P}(\text{A}\cup\text{B})=\text{P}(\text{A}\cap\text{B})$ for any two events A and B, then:

  • A
    $\text{P(A)}=\text{P(B)}$
  • B
    $\text{P(A)}>\text{P(B)}$
  • C
    $\text{P}(\text{A})<\text{P(B)}$
  • D
    none of these.

Answer

  1. $\text{P(A)}=\text{P(B)}$

Solution:

Given that, $\text{P}(\text{A}\cup\text{B})=\text{P}(\text{A}\cap\text{B})$

$\Rightarrow\text{P}(\text{A})+\text{P}(\text{B})-\text{P}(\text{A}\cap\text{B})=\text{P}(\text{A}\cap\text{B})$

$\Rightarrow\big[\text{P(A)}-\text{P}(\text{A}\cap\text{B})\big]+\big[\text{P(B)}-\text{P}(\text{A}\cap\text{B})\big]=0$

But $\text{P(A)}-\text{P}(\text{A}\cap\text{B})\geq0\ ....(\text{i})$

$\big[\because\ \text{P}(\text{A}\cap\text{B})\leq\text{P(A)}\text{ or }\text{P(B)}\big]$

And $\text{P}(\text{B})-\text{P}(\text{A}\cap\text{B})\geq0\ ...(\text{ii})$

From eq. (i) and (ii) we get

$\text{P(A)}=\text{P(B)}$

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