B部分:统计 [60 分]
一组 11 人中包含 6 名男性和 5 名女性,其中 3 人是亲姐妹。现要选出一个由六人组成的委员会。求在下列情况下,有多少种组建委员会的方法:
- (i)
该委员会恰好包含两名男性,
- (ii)
该委员会至少包含姐妹中的一人。
- (iii)
已知选出的委员会由 2 名姐妹 Sue 和 Suzy、3 名男性 Muthu, Mark, Michael 以及 1 名女性 Wina 组成。他们围坐在一张供六人使用的圆桌旁。求在下列情况下,有多少种可能的座位安排:
其中一名男性必须坐在两姐妹之间,
- (iv)
两姐妹恰好坐在彼此正对面。
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完整解法
5(i)
1. Set up the expression to select exactly 2 men and the remaining 4 members from the women
Since the committee requires exactly 2 men (out of 6), the remaining members must be women (out of 5).
2. Evaluate the combinations to find the total number of ways
5(ii)
1. Set up the expression using the complement method by subtracting the number of ways to form a committee with no sisters from the total number of unrestricted ways.
2. Evaluate the combinations to calculate the final number of ways.
434
5(iii)
1. Determine the number of ways to form a block consisting of the two sisters and exactly one man between them.
There are men to choose from to sit between the sisters, so there are choices.
For any chosen man, the two sisters can swap places around him in ways.
Number of ways to form this block = .
2. Treat the block as a single unit and arrange it with the remaining people around the round table.
The block counts as unit. The remaining people ( men and woman) count as units.
Total units to arrange in a circle = .
Number of ways to arrange units in a circle = .
Total possible arrangements = .
5(iv)
1. Seat the two sisters, fixing the circular arrangement
Since it is a circular table, we can fix the position of one sister (e.g., Sue) to remove the rotational symmetry, which gives way.
The other sister (Suzy) must be seated directly opposite her, giving only choice for her seat.
2. Arrange the remaining people
The remaining people (Muthu, Mark, Michael, and Wina) can be arranged in the remaining distinct seats in ways.
Total number of possible arrangements
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