新GRE数学要点:At Least Once

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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.
6 Q7 |: S9 A& u+ B　　Example:
　　To understand the equation follow these steps:　　1.The probability of the event not happening is 1-p.
' |! y" U" _7 p6 L/ t　　2.The probability it won't happen n-times in a row is (1-p)∧n.
; D# H* w3 I: H7 C2 V4 J3 L　　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:
& p) K* L+ G: x5 w9 ]* L　　It is much harder to directly calculate the at-least-once probability.