P1-PS-101

[Huang Hsin-Yi]

101. 10830-!-item-!-187;#058&007415
If xy + z = x(y + z), which of the following must be true?

(A) x = 0 and z = 0
(B) x = 1 and y = 1
(C) y = 1 and z = 0
(D) x = 1 or y = 0
(E) x = 1 or z = 0

Ans:E

請問A與E差別在那呀?

[sunnyblue13s]

題目要must be ture

要先從and 與 or下手判斷

(A)的條件不能充分的驗證 - 不好

[Huang Hsin-Yi]

xy + z = x(y + z)=> z=xz

(a)x=0 則z=0
(e)x=1 則Z不是等於任意數嗎?

[sunnyblue13s]

(E)z=z 他沒有說Z一定要是什麼數~~

(A)你是從結果判斷~~會有失精確....
xy + z = x(y + z)=> xy + z = xy + xz..... z=zx??

[ad264]

這個題目真的很詭異，將(A)代入也是恆等，A為何不true
換句話說，A錯哪？

[Huang Hsin-Yi]

(A)你是從結果判斷~~會有失精確....
xy + z = x(y + z)=> xy + z = xy + xz..... z=zx??

Would you explain more detail?

[sunnyblue13s]

我不太能解釋為什麼(A)一定不能選.....

但就ETS的邏輯，0=0 似乎不是他們想要考的概念

(E) x=1 → y+z=y+z
(E) z=0 → xy=xy

這似乎比較符合考題的概念點~~
GMAT有時候要選的是"比較好"的答案....這點千萬不可以忽略的~~

[Huang Hsin-Yi]

Sorry, I am not conviced.
Help!!!

[chencraig0227]

xy + z = x(y + z)，分解之後，
xy+z=xy+xz 去掉兩邊的xy後，剩下 z=xz
這樣一想不就either x = 1 or z = 0囉!

From: http://www.formosamba.com/phpBB3/viewtopi ... ust+++true

(A)翻中文比較好理解
要把must翻成"必須"
"X一定(must)要等於0; Z一定要(must)等於0"
隨便找個(1,2)就可以weaken (A)
所以(A)錯!

如果題目改成which following could be true, (A)就會對!

[Huang Hsin-Yi]

Thanks! I got the point!!

[ad264]

謝謝樓上各位戰友！

我也發文問了對岸CD，A錯的原因有一個人解釋也不錯。他說是錯在and。
這種題目考場上一定要五個選項都看過，不然用代數看一定上當。