JEE Advance - Mathematics (2012 - Paper 2 Offline - No. 5)
Explanation
Let's analyze the given conditions and evaluate which options are correct.
Given:
- $$ P(X|Y) = \frac{1}{2} $$
- $$ P(Y|X) = \frac{1}{3} $$
- $$ P(X \cap Y) = \frac{1}{6} $$
Now, let's proceed step by step through each option.
Option A: $$ P(X \cup Y) = \frac{2}{3} $$
We can use the formula for the union of two events:
$$ P(X \cup Y) = P(X) + P(Y) - P(X \cap Y) $$
To find $$ P(X) $$ and $$ P(Y) $$, we use the definitions of conditional probability:
$$ P(X|Y) = \frac{P(X \cap Y)}{P(Y)} $$
$$ \frac{1}{2} = \frac{\frac{1}{6}}{P(Y)} $$
$$ P(Y) = \frac{1}{6} \times 2 = \frac{1}{3} $$
Similarly,
$$ P(Y|X) = \frac{P(X \cap Y)}{P(X)} $$
$$ \frac{1}{3} = \frac{\frac{1}{6}}{P(X)} $$
$$ P(X) = \frac{1}{6} \times 3 = \frac{1}{2} $$
Now, substituting these values into the union formula:
$$ P(X \cup Y) = \frac{1}{2} + \frac{1}{3} - \frac{1}{6} $$
To add these fractions, we need a common denominator. The common denominator is 6:
$$ P(X \cup Y) = \frac{3}{6} + \frac{2}{6} - \frac{1}{6} = \frac{4}{6} = \frac{2}{3} $$
Hence, Option A is correct.
Option B: $$ X $$ and $$ Y $$ are independent
For two events $$ X $$ and $$ Y $$ to be independent, the following condition should hold:
$$ P(X \cap Y) = P(X) \times P(Y) $$
We already found that:
$$ P(X) = \frac{1}{2} $$
$$ P(Y) = \frac{1}{3} $$
$$ P(X \cap Y) = \frac{1}{6} $$
Checking the independence condition:
$$ P(X) \times P(Y) = \frac{1}{2} \times \frac{1}{3} = \frac{1}{6} $$
Since this is equal to $$ P(X \cap Y) $$, $$ X $$ and $$ Y $$ are indeed independent.
Hence, Option B is correct.
Option C: $$ X $$ and $$ Y $$ are not independent
From the previous analysis, we have shown that $$ X $$ and $$ Y $$ are independent.
Hence, Option C is not correct.
Option D: $$ P((X^c) \cap Y) = \frac{1}{3} $$
To find $$ P((X^c) \cap Y) $$, we use the fact that:
$$ P(Y) = P((X \cap Y) \cup (X^c \cap Y)) $$
Since $$ (X \cap Y) $$ and $$ (X^c \cap Y) $$ are disjoint events, we can write:
$$ P(Y) = P(X \cap Y) + P(X^c \cap Y) $$
Given:
$$ P(Y) = \frac{1}{3} $$
$$ P(X \cap Y) = \frac{1}{6} $$
Substituting these,
$$ \frac{1}{3} = \frac{1}{6} + P(X^c \cap Y) $$
Solving for $$ P(X^c \cap Y) $$:
$$ P(X^c \cap Y) = \frac{1}{3} - \frac{1}{6} = \frac{2}{6} - \frac{1}{6} = \frac{1}{6} $$
This contradicts Option D, as it says $$ \frac{1}{3} $$.
Hence, Option D is not correct.
In conclusion:
Option A and Option B are correct.
Option C and Option D are not correct.
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