请教一道数学题!!谢谢

cynthia0610

If (y+3)(y-1) – (y-2)(y-1) = r(y-1), what is the value of y?

(1)     r² = 25

(2)     r = 5

                  

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E,我的疑问是当r=-5时,不就可以知道y的值了么?

我觉得是A

请NN们帮忙解答,谢谢

Jeffxf

原式化简后为

（y-1）*5=(y-1)*r

r＝－5确实可以求出y＝1，可是lz有没把y＝1放回去看过呢？当y＝1时，方程变成了恒等式0=0了，也就是说y＝1时r可以等于任何数了，所以是没意义的。

Jeffxf

解方程前先得把未知数的定义域确定了才不容易被坑

cynthia0610

谢谢这位NN

puremilk1120

原来解方程还要注意定义域 谢谢提醒