[問題]GWD10-33

[iamger0730]

Q33:
Is xy > x/y?
(1) xy > 0
(2) y < 0

the answer is E
__________________________

我選A

誰可以救救我

[sunshine88]

Q33:
Is xy > x/y?
(1) xy > 0
(2) y < 0

the answer is E
__________________________

我選A

誰可以救救我

(1)這個條件是不能確定xy > x/y 的
例如x=-1/2, y=-1/4 ==> xy=1/8, x/y=2, xy < x/y
x=2, y=4 ==> xy=8, x/y=1/2, xy > x/y

[FionaV]

請問為什麼條件(2)也是不充分的?

[zazer.lin]

Given y < 0 from statement (2), we can suppose

(i) y = -1, x = 1, then
xy = -1 and x/y = -1
Therefore, xy = x/y

(ii) y = -1/2, x =1, then
xy = -1/2 and x/y = -2
Therefore, xy > x/y

By (i) and (ii) we can get that statement(2) alone is not sufficient.