你有多少种方法可以用两个骰子掷出 8 的总和?
概率意味着可能性。它说明事件即将发生的可能性。事件的概率只能存在于 0 和 1 之间,其中 0 表示事件不会发生,即不可能,1 表示肯定会发生,即确定性。
事件发生的概率越高或越小,事件发生或不发生的可能性就越大。例如 – 一枚无偏的硬币被抛一次。因此,结果的总数只能是 2,即“正面”或“反面”。两种结果的概率相等,即 50% 或 1/2。
因此,事件的概率是有利结果/结果总数。它用括号表示,即P(Event)。
P(Event) = N(Favorable Outcomes) / N (Total Outcomes)
Note: If the probability of occurring of an event A is 1/3 then the probability of not occurring of event A is 1-P(A) i.e. 1- (1/3) = 2/3
什么是样本空间?
事件的所有可能结果称为样本空间。
例子-
- 一个六面骰子掷一次。因此,总结果可以是 6 和
样本空间将是 [1, 2, 3, 4, 5, 6] - 抛一枚无偏的硬币,因此,总结果可以是 2 和
样本空间将是 [Head, Tail] - 如果两个骰子一起滚动,则总结果将为 36 和
样本空间将是
[(1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6)
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6)]
活动类型
独立事件:如果两个事件(A 和 B)是独立的,那么它们的概率为
P(A and B) = P (A ∩ B) = P(A).P(B) i.e. P(A) * P(B)
示例:如果两个硬币被翻转,那么两个硬币都是反面的机会是 1/2 * 1/2 = 1/4
互斥事件:
- 如果事件 A 和事件 B 不能同时发生,则称为互斥事件。
- 如果两个事件互斥,则两者发生的概率记为 P (A ∩ B) 并且
P (A 和 B) = P (A ∩ B) = 0 - 如果两个事件互斥,则任一事件发生的概率记为 P (A ∪ B)
P (A 或 B) = P (A ∪ B)
= P (A) + P (B) - P (A ∩ B)
= P (A) + P (B) - 0
= P (A) + P (B)
示例:在六面骰子上掷出 2 或 3 的机会是 P(2 或 3)= P(2)+ P(3)= 1/6 + 1/6 = 1/3
不互斥事件:如果事件不互斥,则
P (A or B) = P (A ∪ B) = P (A) + P (B) − P (A and B)
什么是条件概率?
对于某个事件 A 的概率,给出了某个其他事件 B 的发生。写成 P (A ∣ B)
P (A ∣ B) = P (A ∩ B) / P (B)
示例-在一袋 3 个黑球和 2 个黄球(共 5 个球)中,拿一个黑球的概率是 3/5,拿第二个球的概率是黑球或黄球取决于先前取出的球。因为,如果拿了一个黑球,那么再次捡到一个黑球的概率是 1/4,因为只剩下 2 个黑球和 2 个黄球,如果之前拿过一个黄球,那么再次捡到一个黑球的概率是黑球将是 3/4。
你有多少种方法可以用两个骰子掷出 8 的总和?
解决方案:
When two dice are rolled together then total outcomes are 36 and
Sample space is
[ (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6)
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6) ]
So, pairs with sum 8 are (2, 6) (3, 5) (4, 4) (5, 3) (6, 2) i.e. total 5 pairs
So, in 5 ways we can roll a 8 with two dice.
类似问题
问题 1:两个骰子掷出 7 有多少种方式?
解决方案:
When two dice are rolled together then total outcomes are 36 and
Sample space is
[ (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6)
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6) ]
So, pairs with sum 7 are (1,6) (2, 5) (3,4) (4, 3) (5, 2) (6,1) i.e. total 6 pairs
So, in 6 ways we can roll a 7 with two dice.
问题 2:用两个骰子掷出 6 有多少种方式?
解决方案:
When two dice are rolled together then total outcomes are 36 and
Sample space is
[ (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6)
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6) ]
So, pairs with sum 6 are (1,5) (2, 4) (3,3) (4, 2) (5, 1) i.e. total 5 pairs
So, in 5 ways we can roll a 6 with two dice.
问题 3:用两个骰子掷出 5 有多少种方式?
解决方案:
When two dice are rolled together then total outcomes are 36 and
Sample space is
[ (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6)
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6)
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6)
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6)
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6)
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6) ]
So, pairs with sum 5 are (1,4) (2, 3) (3,2) (4, 1) i.e. total 4 pairs
So, in 4 ways we can roll a 5 with two dice.