2023 AIME I 第 6 题

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6.

Alice 知道将有 33 张红牌和 33 张黑牌按随机顺序一张一张展示给她。每张牌展示前,Alice 必须猜它的颜色。 如果 Alice 采用最优策略,她猜对牌数的期望为 mn\frac{m}{n},其中 mmnn 是互质的正整数。求 m+nm + n

Alice knows that 33 red cards and 33 black cards will be revealed to her one at a time in random order. Before each card is revealed, Alice must guess its color. If Alice plays optimally, the expected number of cards she will guess correctly is mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+n.m + n.

答案:51
知识点:期望值递推概率最优化
难度评级:2600
解答:

无论 Alice 猜什么颜色,牌堆接下来的状态转移方式都一样;只有这一步猜中的概率取决于她的猜测,因此最优做法是猜剩余张数最多的颜色。 令 E(r,b)E(r, b) 表示还剩 rr 张红牌和 bb 张黑牌时,之后猜对张数的期望。则 E(r,0)=rE(r, 0) = rE(0,b)=bE(0, b) = b,且 E(r,b)=max(r,b)r+b+rE(r1,b)r+b+bE(r,b1)r+b. \begin{aligned} E(r, b) &= \frac{\max(r, b)}{r + b} \\ &\quad {}+ \frac{r\,E(r-1,b)}{r+b} \\ &\quad {}+ \frac{b\,E(r,b-1)}{r+b}. \end{aligned}

由对称性 E(r,b)=E(b,r)E(r, b) = E(b, r)。向上计算: E(1,1)=32E(1,1) = \frac{3}{2}E(2,1)=23+232+23=73E(2,1) = \frac{2}{3} + \frac{2 \cdot \frac{3}{2} + 2}{3} = \frac{7}{3}E(2,2)=12+73=176E(2,2) = \frac{1}{2} + \frac{7}{3} = \frac{17}{6}E(3,1)=34+373+34=134E(3,1) = \frac{3}{4} + \frac{3 \cdot \frac{7}{3} + 3}{4} = \frac{13}{4}E(3,2)=35+3176+21345=185E(3,2) = \frac{3}{5} + \frac{3 \cdot \frac{17}{6} + 2 \cdot \frac{13}{4}}{5} = \frac{18}{5}, 最后 E(3,3)=12+185=4110E(3,3) = \frac{1}{2} + \frac{18}{5} = \frac{41}{10}

因此猜对张数的期望为 4110\frac{41}{10},所以 m+n=41+10=51m + n = 41 + 10 = 51

Whatever Alice guesses, the deck evolves the same way; only the immediate success probability depends on her guess, so it is optimal to guess a color with the most cards remaining. Let E(r,b)E(r, b) be the expected number of correct guesses from a state with rr red and bb black cards left. Then E(r,0)=r,E(r, 0) = r, E(0,b)=b,E(0, b) = b, and E(r,b)=max(r,b)r+b+rE(r1,b)r+b+bE(r,b1)r+b. \begin{aligned} E(r, b) &= \frac{\max(r, b)}{r + b} \\ &\quad {}+ \frac{r\,E(r-1,b)}{r+b} \\ &\quad {}+ \frac{b\,E(r,b-1)}{r+b}. \end{aligned}

By symmetry E(r,b)=E(b,r).E(r, b) = E(b, r). Computing upward: E(1,1)=32,E(1,1) = \frac{3}{2}, E(2,1)=23+232+23=73,E(2,1) = \frac{2}{3} + \frac{2 \cdot \frac{3}{2} + 2}{3} = \frac{7}{3}, E(2,2)=12+73=176,E(2,2) = \frac{1}{2} + \frac{7}{3} = \frac{17}{6}, E(3,1)=34+373+34=134,E(3,1) = \frac{3}{4} + \frac{3 \cdot \frac{7}{3} + 3}{4} = \frac{13}{4}, E(3,2)=35+3176+21345=185,E(3,2) = \frac{3}{5} + \frac{3 \cdot \frac{17}{6} + 2 \cdot \frac{13}{4}}{5} = \frac{18}{5}, and finally E(3,3)=12+185=4110.E(3,3) = \frac{1}{2} + \frac{18}{5} = \frac{41}{10}.

So the expected number of correct guesses is 4110,\frac{41}{10}, and m+n=41+10=51.m + n = 41 + 10 = 51.

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