数学题求帮忙

dreamworlds

Q6:
If x and y are positive, what is the value of x – y?
(1)   (x² – y²)/ (x + y) = 4
(2)   x + y = 7
                 
A. Statement (1) ALONE is sufficient, but statement(2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement(1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient,but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.
答案是A。


请指点。

yayafl

非牛人个人想法：
1:（x+y)(x-y)/(x+y)=4 -->(x-y)=4 到此，充分。
2：只有（x+y)条件不全，于是 不充分

So, A.

dreamworlds

谢谢了。刚刚查到你是对的。我不知道这个定理。。

gesteven

我做prep的时候为什么说这题的答案是C呢？