要过程
參考答案:图象经过(-2,1),(1,1),(-1,-1),
即x=-2,y=1; x=1,y=1;x=-1,y=-1;
所以写出三个方程式:
1 = a * (-2)^2 + b*(-2) + c
1 = a * (1)^2 + b *(1) + c
-1 = a * (-1)^2 + b*(-1) + c
得:
1 = 4a -2b + c (式1)
1 = a + b + c (式2)
-1 = a - b + c (式3)
(式2)减(式3)得 2=2b, 所以 b = 1
把b = 1 代入 (式1)(式2)
1 = 4a -2 + c
1 = a + 1 + c
两式相减 得 0 = 3a -3, 所以 a = 1
把b = 1 , a = 1 代入 (式3)得 c = -1;
函数解析式: y = x^2 + x -1
顶点坐标:
xt= -b/(2a) = -0.5;
yt = 0.25 - 0.5 - 1 = -1.25