In a tournament, a team plays 10 matches with probabilities of winning and losing each match as $\frac{1}{3}$ and $\frac{2}{3}$ respectively. Let $x$ be the number of matches that the team wins, and $y$ be the number of matches that team loses. If the probability $\mathrm{P}(|x-y| \leq 2)$ is $p$, then $3^9 p$ equals _________.
Answer (integer)
8288
Solution
<p>$$\begin{aligned}
& x+y=10 \\
& A=x-y \\
& P(|A|<2) \text { is } P \\
& \Rightarrow|A|=2,1,0 \Rightarrow A=0,1,-1,2,-2 \\
& \Rightarrow x=\frac{10+A}{2} \Rightarrow A \in \text { even as } x \in \text { integer } \\
& \Rightarrow A=0,-2,2 \\
& \Rightarrow P(|A| \leq 2)=P(A=0)+P(A=-2)+P(A=2)
\end{aligned}$$</p>
<p>(1) $A=0 \Rightarrow x=5=y$</p>
<p>$P(A=0)={ }^{10} C_5\left(\frac{1}{3}\right)^5\left(\frac{2}{3}\right)^5$</p>
<p>(2) $A=-2$</p>
<p>$\Rightarrow x=4$ and $y=6$</p>
<p>$P(A=-2)={ }^{10} C_4 \cdot\left(\frac{1}{3}\right)^4\left(\frac{2}{3}\right)^6$ and</p>
<p>$$\begin{aligned}
& \text { Similarly, } P(A=2)={ }^{10} C_6\left(\frac{1}{3}\right)^6\left(\frac{2}{3}\right)^4 \\
& \Rightarrow P(|A| \leq 2) 3^9=3\left({ }^{10} C_5 \cdot 2^5+{ }^{10} C_4 \cdot 2^6+{ }^{10} C_6 \cdot 2^4\right) \\
& =8288
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Probability · Topic: Classical and Axiomatic Probability
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