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标题: 求助 prep 题目太短了搜不到··· [打印本页]

作者: rx_11 时间: 2010-7-16 14:28
标题: 求助 prep 题目太短了搜不到···
is xy>o?
(1) x-y>-2
(2) x-2y<-6

完全没思路。。。求解···

作者: youngerlee 时间: 2010-7-16 14:33
xy>0

means x>0 y>0 or x<0 y<0

(1) x-y>-2

insuffcient when x=-1 y=0

(2) x-2y<-6

insufficient when x=-10 y=1

[1]+[2]

2y-6>x>y-2
which means y>4 x>2

C?

作者: rx_11 时间: 2010-7-16 14:43
2y-6>x>y-2
which means y>4 x>2

弱问``how do you know y>4 and x>2 from the condition above?

作者: youngerlee 时间: 2010-7-16 14:44
2y-6>x>y-2

可以得出

2y-6必定比y-2大

然后通分一下

作者: rx_11 时间: 2010-7-16 14:45

泪流满面```好聪明啊```

作者: gracezz 时间: 2010-7-16 14:45
C?

作者: rx_11 时间: 2010-7-16 14:46
恩``C

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