题目
A housing estate consists of 320 houses. A house on the estate is selected at random.
denotes the event ‘the house has a driveway’
denotes the event ‘no cars per household’
denotes the event ‘exactly 1 car per household’
denotes the event ‘exactly 2 cars per household’
The Venn diagram shows the number of households in each region for the events , , and .
(a) Find (i)
(ii)
(iii)
Given that the house has a driveway,
(b) find the probability that there are exactly 2 cars in the household.
(c) Write down 2 of the events , , and that are mutually exclusive.
(d) Determine whether the events and are independent. Justify your answer.
(e) Define in the context of this question, the event
(i)
(ii)
解答
(a)
解法一
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直接从 Venn 图加区域。第三问用补集更快:。
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From the diagram,
Also,
For , use the complement:
Since ,
(b)
解法一
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已知有 driveway,所以分母是 中的 200 户;其中 exactly 2 cars 是 的 88 户。
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(c)
解法一
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没有车、1 辆车、2 辆车这些事件彼此不能同时发生。
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and are mutually exclusive.
(d)
解法一
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可以比较 与 。如果相等,则独立。
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From the diagram,
Also,
Since
the events and are independent.
(e)
解法一
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把集合符号逐个翻译成语句即可。 是没有 driveway, 是 exactly 2 cars。
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means:
The house does not have a driveway and has exactly 2 cars.
means:
The house has a driveway and has fewer than 2 cars.