Q19
17 marks
Binomial Distribution
Contextual Probability Requiring Binomial Modeling Setup
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Two people, A and B, practice table tennis. The winner of each ball scores 1 point, the loser scores 0 points. Let the probability that A wins each ball be $p$ $\left(\frac{1}{2} < p < 1\right)$, the probability that B wins be $q$, with $p + q = 1$. The outcome of each ball is independent. For a positive integer $k \geq 2$, let $p_k$ denote the probability that after $k$ balls, A has scored at least 2 more points than B, and let $q_k$ denote the probability that after $k$ balls, B has scored at least 2 more points than A.
(1) Find $p_3, p_4$ (expressed in terms of $p$).
(2) If $\frac{p_4 - p_3}{q_4 - q_3} = 4$, find $p$.
(3) Prove: For any positive integer $m$, $p_{2m+1} - q_{2m+1} < p_{2m} - q_{2m} < p_{2m+2} - q_{2m+2}$.