Probability
Independent Events and Subset Selection
Grade 12

Question:

<p>(C) \(P \cap Q = \phi\) is</p>
<p>(P) \(\frac{\binom{2n}{n}}{4^n}\)</p>
<p>(Q) \(\frac{2^{2n} - \binom{2n}{n}}{2^{2n+1}}\)</p>
<p>(R) \(\frac{\binom{2n}{n+1}}{4^n}\)</p>
<p>(S) \(\left(\frac{3}{4}\right)^n\)</p>
<p>(T) \(\frac{\binom{2n}{n}}{4^{n-1}}\)</p>

Step-by-Step Solution

Key Concept: For disjoint sets, each element has exactly 3 valid states instead of 4.
<p>For each element $a \in A$, it can be in one of four states: (1) in neither <i>P</i> nor <i>Q</i>, (2) in <i>P</i> only, (3) in <i>Q</i> only, (4) in both <i>P</i> and <i>Q</i>.</p><p>For $P \cap Q = \phi$, no element can be in both. So each element has only 3 choices: not in <i>P</i> or <i>Q</i>, in <i>P</i> only, or in <i>Q</i> only.</p><p>Number of favorable outcomes: $3^n$</p><p>Probability: $\frac{3^n}{4^n} = \left(\frac{3}{4}\right)^n$</p>
Correct Answer: S

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