问一下关于一条prep的数学题

dxt678

选C吧，联合条件可证明不在同一象限

mirrorimage

since ab!=0 and (-a,b) and (-b,a) are in the same quadrant,
there are two posibilities,
both a,b are +ve or both a,b are -ve.
1) xy>0.
this implies both x,y are positive or both are negative.
so we cant guess whether it will lie in same quadrant or not.
a, b can both be +ve and x,y both can be-ve. so Insufficient.
2) ax>0.
In this case both can be +ve or both can be -ve.
But we dont kno abt sign of Y, so insufficient.
combining 1 and 2.
say both a and x are positive.
Then as x is positive Y has to be positive to satisfy 1, also as a is +ve B is also.
So can be answerd.
same is the case when a is -ve.

So answer is C

Heather_Lau

为什么我觉得1是suffienct的？
根据题干-a*b>0，(1)里面说xy>0,那么-xy<0,那么肯定(-x,y)跟(-a,b) or(b,-a)不在同一个象限咯。

dxt678

为什么我觉得1是suffienct的？
根据题干-a*b>0，(1)里面说xy>0,那么-xy<0,那么肯定(-x,y)跟(-a,b) or(b,-a)在同一个象限咯。

-- by 会员 Heather_Lau (2011/7/5 13:08:15)

反例
假设a=2, b=-2, 则（-a, b)在第三象限 (-2,-2)

x=-2, y=-2    则（-x,y)在第四象限 （2，-2）

julie12231223

billxxyy

平均每个group 12,000个人，9个group不变，把一个group的人移到另一个group，设为a和b，a*1.1 = b，a+b = 132,000 - 12,000*9=24,000，这样算下来a可以有11400+个人。这样我选择e。请大家更正。

Heather_Lau

为什么我觉得1是suffienct的？
根据题干-a*b>0，(1)里面说xy>0,那么-xy<0,那么肯定(-x,y)跟(-a,b) or(b,-a)在同一个象限咯。

-- by 会员 Heather_Lau (2011/7/5 13:08:15)

反例
假设a=2, b=-2, 则（-a, b)在第三象限 (-2,-2)

x=-2, y=-2    则（-x,y)在第四象限 （2，-2）

-- by 会员 dxt678 (2011/7/5 13:36:10)

说错了，应该是肯定不在同一个象限。
召唤LZ，告知一下正确答案，我觉得是A啊。

lastjuly

THX~~~~~

fordmilan

C吧