求助 prep 题目太短了搜不到···

rx_11

is xy>o?
(1)  x-y>-2
(2)  x-2y<-6

完全没思路。。。求解···

youngerlee

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?

rx_11

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?

youngerlee

2y-6>x>y-2

可以得出

2y-6必定比y-2大

然后通分一下

rx_11

泪流满面```好聪明啊```

gracezz

C?

rx_11

恩``C