四面体骰子有四个面。两个均匀的四面体骰子,一红一蓝,其四个面分别标有数字 0、1、2 和 3。投掷这两个骰子,并记录两个骰子朝下的数字。随机变量 TT 定义为红色骰子上的得分乘以蓝色骰子上的得分。

  1. (i)

    求 TT 的概率分布。

  2. (ii)

    求 E(T)E(T) 并证明 Var(T)=11516\mathrm{Var}(T)=\dfrac{115}{16}。清楚地写出计算步骤。

  3. (iii)

    计算 P(∣T−2μ∣>σ)P(|T-2\mu|>\sigma) 的值,其中 μ=E(T)\mu=E(T) 且 σ2=Var(T)\sigma^2=\mathrm{Var}(T)。

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完整解法

6(i)

1. Construct a table of outcomes to find all possible values of $T$.

The possible scores on each die are 0,1,2,30, 1, 2, 3. There are 4×4=164 \times 4 = 16 equally likely outcomes for the pair of dice.
The product of the scores, TT, can be represented in a table of outcomes:

×012300000101232024630369\begin{array}{c|cccc} \times & 0 & 1 & 2 & 3 \\ \hline 0 & 0 & 0 & 0 & 0 \\ 1 & 0 & 1 & 2 & 3 \\ 2 & 0 & 2 & 4 & 6 \\ 3 & 0 & 3 & 6 & 9 \end{array}

2. Calculate the probability for each value of $T$.

By counting the frequencies of each possible value of TT out of the 1616 total outcomes, we have:
P(T=0)=716P(T=0) = \frac{7}{16}
P(T=1)=116P(T=1) = \frac{1}{16}
P(T=2)=216=18P(T=2) = \frac{2}{16} = \frac{1}{8}
P(T=3)=216=18P(T=3) = \frac{2}{16} = \frac{1}{8}
P(T=4)=116P(T=4) = \frac{1}{16}
P(T=6)=216=18P(T=6) = \frac{2}{16} = \frac{1}{8}
P(T=9)=116P(T=9) = \frac{1}{16}

3. Tabulate the probability distribution of $T$.

Summarising the probabilities into a formal distribution table:

t0123469P(T=t)716116181811618116\begin{array}{|c|c|c|c|c|c|c|c|} \hline t & 0 & 1 & 2 & 3 & 4 & 6 & 9 \\ \hline P(T=t) & \frac{7}{16} & \frac{1}{16} & \frac{1}{8} & \frac{1}{8} & \frac{1}{16} & \frac{1}{8} & \frac{1}{16} \\ \hline \end{array}
答案
t0123469P(T=t)716116181811618116\begin{array}{|c|c|c|c|c|c|c|c|} \hline t & 0 & 1 & 2 & 3 & 4 & 6 & 9 \\ \hline P(T=t) & \frac{7}{16} & \frac{1}{16} & \frac{1}{8} & \frac{1}{8} & \frac{1}{16} & \frac{1}{8} & \frac{1}{16} \\ \hline \end{array}

6(ii)

1. Determine the probability distribution of $T$ (from part (i)) and calculate the expected value $E(T)$.

The possible outcomes for T=X×YT = X \times Y where X,Y∈{0,1,2,3}X, Y \in \{0, 1, 2, 3\} give the following probability distribution:
P(T=0)=716P(T=0) = \frac{7}{16}, P(T=1)=116P(T=1) = \frac{1}{16}, P(T=2)=216P(T=2) = \frac{2}{16}, P(T=3)=216P(T=3) = \frac{2}{16}, P(T=4)=116P(T=4) = \frac{1}{16}, P(T=6)=216P(T=6) = \frac{2}{16}, P(T=9)=116P(T=9) = \frac{1}{16}.

E(T)=∑tP(T=t)=0(716)+1(116)+2(216)+3(216)+4(116)+6(216)+9(116)=0+1+4+6+4+12+916=3616=94\begin{aligned} E(T) &= \sum t P(T=t) \\ &= 0\left(\frac{7}{16}\right) + 1\left(\frac{1}{16}\right) + 2\left(\frac{2}{16}\right) + 3\left(\frac{2}{16}\right) + 4\left(\frac{1}{16}\right) + 6\left(\frac{2}{16}\right) + 9\left(\frac{1}{16}\right) \\ &= \frac{0 + 1 + 4 + 6 + 4 + 12 + 9}{16} \\ &= \frac{36}{16} \\ &= \frac{9}{4} \end{aligned}

2. Calculate $E(T^2)$ and apply the variance formula $\mathrm{Var}(T) = E(T^2) - [E(T)]^2$ to show the required result.

E(T2)=∑t2P(T=t)=02(716)+12(116)+22(216)+32(216)+42(116)+62(216)+92(116)=0+1+8+18+16+72+8116=19616=494\begin{aligned} E(T^2) &= \sum t^2 P(T=t) \\ &= 0^2\left(\frac{7}{16}\right) + 1^2\left(\frac{1}{16}\right) + 2^2\left(\frac{2}{16}\right) + 3^2\left(\frac{2}{16}\right) + 4^2\left(\frac{1}{16}\right) + 6^2\left(\frac{2}{16}\right) + 9^2\left(\frac{1}{16}\right) \\ &= \frac{0 + 1 + 8 + 18 + 16 + 72 + 81}{16} \\ &= \frac{196}{16} \\ &= \frac{49}{4} \end{aligned}

Var(T)=E(T2)−[E(T)]2=494−(94)2=19616−8116=11516\begin{aligned} \mathrm{Var}(T) &= E(T^2) - [E(T)]^2 \\ &= \frac{49}{4} - \left(\frac{9}{4}\right)^2 \\ &= \frac{196}{16} - \frac{81}{16} \\ &= \frac{115}{16} \end{aligned}

答案

E(T)=94E(T) = \frac{9}{4}

6(iii)

1. Substitute the values of $\mu$ and $\sigma$ to form an inequality for $T$.

From part (ii), we have E(T)=μ=94=2.25E(T) = \mu = \frac{9}{4} = 2.25 and Var(T)=11516\mathrm{Var}(T) = \frac{115}{16}.
Thus, σ=11516=1154≈2.681\sigma = \sqrt{\frac{115}{16}} = \frac{\sqrt{115}}{4} \approx 2.681.
The expression ∣T−2μ∣>σ|T - 2\mu| > \sigma becomes:
∣T−2(2.25)∣>2.681|T - 2(2.25)| > 2.681
∣T−4.5∣>2.681|T - 4.5| > 2.681

2. Solve the modulus inequality to find the range of valid values for $T$.

T−4.5<−2.681orT−4.5>2.681T - 4.5 < -2.681 \quad \text{or} \quad T - 4.5 > 2.681
T<1.819orT>7.181T < 1.819 \quad \text{or} \quad T > 7.181

3. Identify the values of $T$ from its probability distribution that satisfy the inequality.

The possible values of TT (the product of the scores on the two dice) are 0,1,2,3,4,6,0, 1, 2, 3, 4, 6, and 99.
The values of TT that satisfy T<1.819T < 1.819 or T>7.181T > 7.181 are 0,1,0, 1, and 99.

4. Calculate the total probability for the identified values of $T$.

P(∣T−2μ∣>σ)=P(T=0)+P(T=1)+P(T=9)\mathrm{P}(|T - 2\mu| > \sigma) = \mathrm{P}(T = 0) + \mathrm{P}(T = 1) + \mathrm{P}(T = 9)
Using the probabilities of outcomes:
=716+116+116= \frac{7}{16} + \frac{1}{16} + \frac{1}{16}
=916= \frac{9}{16}

答案

916\frac{9}{16}

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