The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively.What value is exactly 2 standard deviations less than the mean?
(A) 10.5
(B)11.0
(C)11.5
(D) 12.0
(E) 12.5
If two of the four expressions x + y, x + 5y, x - y, and 5x - y are chosen at random, what is the probability that their product will be of the form of x^2 - (by)^2, where b is an integer? my wrong answer is 1/2, please advise。。。C(1,3)/C(2,4)
The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively.What value is exactly 2 standard deviations less than the mean?
If two of the four expressions x + y, x + 5y, x - y, and 5x - y are chosen at random, what is the probability that their product will be of the form of x^2 - (by)^2, where b is an integer? my wrong answer is 1/2, please advise。。。C(1,3)/C(2,4)
x^2-(by)^2=(x-by)*(x+by), so the only suitable answer is x+y and x-y ,since the total option for the given expressions is C(2,4),the correct answer is 1/c(2,4)=1/6
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发表于 2007-11-24 06:12:00
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