Prep 上的三道题。 先谢谢了
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 x <> -y, is x+y/x-y > 1
(1) x > 0
(2) y < 0
If x and y are positive integers such that x = 8y + 12, what is the greatest common divisor of x and y ?
(1) x = 12u, where u is an integer.
(2) y = 12z, where z is an integer.
Prep 上的三道题。 先谢谢了
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 x <> -y, is x+y/x-y > 1
(1) x > 0
(2) y < 0
If x and y are positive integers such that x = 8y + 12, what is the greatest common divisor of x and y ?
(1) x = 12u, where u is an integer.
(2) y = 12z, where z is an integer.
第一题，13.5-2*1.5=10.5 选A
第二题，x <> -y是不是打错了
第三题，（1）plugging x=12u back to x=8y+12 we may get y=(3/2)*(u-1) where y is a positive integer, since x=12u is also a positive integer, we may find out the greatest common divisor of x and y could be 3,6,9,12
（2）plugging y=12z back to x=8y+12 we may get x=12(8z+1) where x is a positive integer, since y=12z is also a positive integer, we may find out the greatest common divisor of x and y could be 12 only.
选B
很困，如果算的不对，请谅解。
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