prep ds 1 44

Kath1985

44.3852-!-item-!-187;#058&003578

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).

-------------------------------------------------------------------------------------------------------------------------------【答案】C



題目已說拋物線和x軸有兩交點(即方程式y=0) 即x^+(a+b)x+ab=0

條件2 將0,-6代入x^+(a+b)x+ab=y

我將0,-6代入x^+(a+b)x+ab=0因y本身已經=0了(因拋物線和x軸有兩交點y必為0))



請問我的觀念哪裡錯了

wyjolivia

(0,-6),是抛物线与y轴的交点，所以这点带入x+(a+b)x+ab=Y，————0+(a+b)*0+ab= -6, 得到a+b=-1, and ab=-6, so a=-3, b=2
方程式 y=(x-3)(x+2), 在x轴的交点是，(3,0) (-2,0)