在本题中,你应该清晰地写明所使用的任何分布的相应参数值。

  1. ((a))

    足球的直径服从正态分布,均值为 22.0 cm,标准差为 0.3 cm;而篮球的直径服从正态分布,均值为 μ\mu cm,标准差为 0.4 cm。

    随机挑选的一个篮球的直径比随机挑选的一个足球的直径至少大 10% 的概率为 0.35。

    1. ((i))

      证明 μ=24.0\mu = 24.0。

    2. ((ii))

      画出草图以展示随机挑选的篮球直径的分布,包括曲线的主要特征。在你的草图上,用阴影表示随机挑选的篮球直径在 22.8 cm 到 25.2 cm 之间的概率区域。

    3. ((iii))

      求10个随机选择的足球的平均直径与一个随机选择的篮球的直径之差小于 1.5 cm 的概率。

  2. ((b))

    一款电脑游戏模拟了篮球运动员如何通过向高处的篮圈投篮来得分。球员通过使用鼠标改变投篮角度来进行瞄准。该球员在第一次投篮中得分的概率是 511\dfrac{5}{11},在第二次投篮中得分的概率是 58\dfrac{5}{8}。

    1. ((i))

      证明该选手在两次尝试中均得分的概率至少为 788\dfrac{7}{88}。

    2. ((ii))

      该选手在第一次尝试中得分但在第二次尝试中未得分的概率为 1588\dfrac{15}{88}。求该选手在两次尝试中均得分的概率。事件“选手在第一次尝试中得分”与“选手在第二次尝试中得分”是否独立?请说明理由。

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完整解法

10((a))((i))

1. Define the appropriate linear combination and find its mean and variance.

Let BB and FF be the diameters of a basketball and football respectively.
B∼N(μ,0.42)B \sim \mathrm{N}(\mu, 0.4^2) and F∼N(22.0,0.32)F \sim \mathrm{N}(22.0, 0.3^2).
Let X=B−1.1FX = B - 1.1F.
E(X)=E(B)−1.1E(F)=μ−1.1(22.0)=μ−24.2\mathrm{E}(X) = \mathrm{E}(B) - 1.1\mathrm{E}(F) = \mu - 1.1(22.0) = \mu - 24.2
Var(X)=Var(B)+1.12Var(F)=0.42+1.21(0.32)=0.16+0.1089=0.2689\mathrm{Var}(X) = \mathrm{Var}(B) + 1.1^2\mathrm{Var}(F) = 0.4^2 + 1.21(0.3^2) = 0.16 + 0.1089 = 0.2689

2. Formulate the probability equation and use the inverse normal distribution.

Given P(B≥1.1F)=0.35\mathrm{P}(B \ge 1.1F) = 0.35, we have P(X≥0)=0.35\mathrm{P}(X \ge 0) = 0.35.
Standardising XX:
P(Z≥0−(μ−24.2)0.2689)=0.35\mathrm{P}\left(Z \ge \frac{0 - (\mu - 24.2)}{\sqrt{0.2689}}\right) = 0.35
From the standard normal distribution (inverse normal):
24.2−μ0.2689≈0.38532\frac{24.2 - \mu}{\sqrt{0.2689}} \approx 0.38532

3. Solve the equation for $\mu$.

24.2−μ=0.38532×0.268924.2 - \mu = 0.38532 \times \sqrt{0.2689}
24.2−μ≈0.1998124.2 - \mu \approx 0.19981
μ=24.2−0.19981=24.00019\mu = 24.2 - 0.19981 = 24.00019
μ≈24.0 (shown)\mu \approx 24.0 \text{ (shown)}

答案

μ=24.0\mu = 24.0

10((a))((ii))

1. Identify the parameters of the normal curve and the significance of the boundary values.

Let the diameter of a basketball be YY. From part (a)(i), Y∼N(24.0,0.42)Y \sim \mathrm{N}(24.0, 0.4^2).
The curve is centered at the mean, μ=24.0\mu = 24.0.
The bounds for shading are 22.822.8 and 25.225.2.
Notice that 22.8=24.0−3(0.4)22.8 = 24.0 - 3(0.4) and 25.2=24.0+3(0.4)25.2 = 24.0 + 3(0.4), corresponding exactly to μ−3σ\mu - 3\sigma and μ+3σ\mu + 3\sigma.

2. Describe the features of the sketch and the shaded region.

To draw the sketch:

  1. Draw a symmetrical, bell-shaped curve above a horizontal axis.
  2. Mark the center of the peak on the axis as 24.024.0.
  3. Mark 22.822.8 and 25.225.2 symmetrically on the left and right of 24.024.0.
  4. Ensure the tails of the curve asymptotically approach the horizontal axis near and beyond these values.
  5. Shade the entire area under the curve between x=22.8x = 22.8 and x=25.2x = 25.2.
答案

A symmetrical bell-shaped curve with its peak at x=24.0x = 24.0 on the horizontal axis, asymptotes at both tails, and the region under the curve between x=22.8x = 22.8 and x=25.2x = 25.2 shaded.

10((a))((iii))

1. State the distributions of the sample mean of 10 footballs and the basketball diameter.

Let FF be the diameter of a football and BB be the diameter of a basketball.
From the given information and part (a)(i):
B∼N(24.0,0.42)B \sim \mathrm{N}(24.0, 0.4^2)
Let Fˉ\bar{F} be the average diameter of 10 footballs.
Fˉ∼N(22.0,0.3210)  ⟹  Fˉ∼N(22.0,0.009)\bar{F} \sim \mathrm{N}\left(22.0, \frac{0.3^2}{10}\right) \implies \bar{F} \sim \mathrm{N}(22.0, 0.009)

2. Define the difference variable and find its distribution.

Let W=B−FˉW = B - \bar{F}.
E(W)=E(B)−E(Fˉ)=24.0−22.0=2.0\mathrm{E}(W) = \mathrm{E}(B) - \mathrm{E}(\bar{F}) = 24.0 - 22.0 = 2.0
Since BB and Fˉ\bar{F} are independent normal variables, we add their variances:
Var(W)=Var(B)+Var(Fˉ)=0.42+0.009=0.16+0.009=0.169\mathrm{Var}(W) = \mathrm{Var}(B) + \mathrm{Var}(\bar{F}) = 0.4^2 + 0.009 = 0.16 + 0.009 = 0.169
Therefore, W∼N(2.0,0.169)W \sim \mathrm{N}(2.0, 0.169).

3. Evaluate the probability that the absolute difference is less than 1.5 cm.

The difference between the diameters is less than 1.5 cm1.5\text{ cm}, which means the absolute difference ∣W∣<1.5|W| < 1.5.
P(∣W∣<1.5)=P(−1.5<W<1.5)\mathrm{P}(|W| < 1.5) = \mathrm{P}(-1.5 < W < 1.5)
Using a graphing calculator:
P(−1.5<W<1.5)≈0.11194≈0.112 (to 3 s.f.)\mathrm{P}(-1.5 < W < 1.5) \approx 0.11194 \approx 0.112\text{ (to 3 s.f.)}

答案

0.1120.112

10((b))((i))

1. Set up the inequality using the addition rule and the maximum value of the union of probabilities.

Let AA be the event of scoring on the first attempt and BB be the event of scoring on the second attempt.
Using the addition rule for probabilities:
P(A∪B)=P(A)+P(B)−P(A∩B)P(A \cup B) = P(A) + P(B) - P(A \cap B)
Since the probability of any event cannot exceed 1, we know that P(A∪B)≤1P(A \cup B) \le 1.
Therefore, P(A)+P(B)−P(A∩B)≤1P(A) + P(B) - P(A \cap B) \le 1.

2. Substitute the given probabilities and simplify to show the required minimum value.

Rearranging the inequality gives:
P(A∩B)≥P(A)+P(B)−1P(A \cap B) \ge P(A) + P(B) - 1
Substituting the given values P(A)=511P(A) = \frac{5}{11} and P(B)=58P(B) = \frac{5}{8}:
P(A∩B)≥511+58−1P(A \cap B) \ge \frac{5}{11} + \frac{5}{8} - 1
P(A∩B)≥4088+5588−8888P(A \cap B) \ge \frac{40}{88} + \frac{55}{88} - \frac{88}{88}
P(A∩B)≥9588−8888P(A \cap B) \ge \frac{95}{88} - \frac{88}{88}
P(A∩B)≥788 (shown)P(A \cap B) \ge \frac{7}{88} \text{ (shown)}

答案

Shown

10((b))((ii))

1. Find the probability of scoring on both attempts using the relationship between events.

Let FF be the event the player scores on the first attempt and SS be the event the player scores on the second attempt.
The probability of scoring on the first attempt but not on the second is P(F∩S′)P(F \cap S').
We know that P(F)=P(F∩S)+P(F∩S′)P(F) = P(F \cap S) + P(F \cap S').
P(F∩S)=P(F)−P(F∩S′)P(F \cap S) = P(F) - P(F \cap S')
P(F∩S)=511−1588P(F \cap S) = \frac{5}{11} - \frac{15}{88}
P(F∩S)=4088−1588=2588P(F \cap S) = \frac{40}{88} - \frac{15}{88} = \frac{25}{88}

2. Check the condition for independence by comparing the product of individual probabilities with the probability of the intersection.

For events FF and SS to be independent, P(F∩S)P(F \cap S) must equal P(F)×P(S)P(F) \times P(S).
P(F)×P(S)=511×58=2588P(F) \times P(S) = \frac{5}{11} \times \frac{5}{8} = \frac{25}{88}
Since P(F∩S)=P(F)×P(S)=2588P(F \cap S) = P(F) \times P(S) = \dfrac{25}{88}, the events are independent.

答案

Probability of scoring on both attempts is 2588\dfrac{25}{88}. Yes, the events are independent because P(first)×P(second)=P(both)P(\text{first}) \times P(\text{second}) = P(\text{both}).

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