Prep-T1-DS 44

[hsinyang]

44.
In the xy-plane, at what two points does the graph of y = (x + a)(x + b) intersect the x-axis?

(1) a + b = -1

(2) The graph intersects the y-axis at (0, -6).


=>(1)答案不就可以求出2個點嗎?爲什麼答案是C

請問有誰可以幫我解答一下，謝謝!

[scottkidd]

y = (x + a)(x + b)
==> y=x^2+(a+b)x+ab

(1) a + b = -1
==> 可把原題變成 y=x^2-x+ab，不知道ab多少，insufficient

(2) The graph intersects the y-axis at (0, -6)
==> 可把原題變成 -6=0-0+ab，再代回就可以得到完整的方程式，這樣也才sufficient

[hsinyang]

謝謝你!
我知道我哪裡弄錯了