GRE数学重要知识点：AT least One

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　　At Least Once

　　If probability of an event happening is p, then the probability that the event happens at least once in n trials is 1-(1-p)∧n.

　　Example:

　　To understand the equation follow these steps:

　　1.The probability of the event not happening is 1-p.

　　2.The probability it won&apost happen n-times in a row is (1-p)∧n.

　　3.Probability it will happen at least once is 1-(1-p)∧n.

　　Probability that a roll of a six-sided die land with face 1 is 1/6.Then the probability that 1 will turn up at least once in 3 tries:=1-(1-1/6)∧3=1-(5/6)∧3.

　　Remember:

　　It is much harder to directly calculate the at-least-once probability.