请教3道数学题

illfly

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.

dianecarton

以下是引用illfly在2008-3-3 3:42:00的发言：

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。