(1)
直線 y = kx - 4k = k(x - 4)
A(4, 0), B(0, -4k)
C(-10, 0), 對稱軸x = (-10 + 4)/2 = -3
y = a(x + 10)(x - 4)
x = 0, y = -40a = -4k, k = 10a
tan∠DBO = 3/4 = DO/OB = 3/(-4k)
k = -1
a = -1/10
拋物線的解析式: y = -(x + 10)(x - 4)/10 = -x?/10 - 3x/5 + 4
(2)
對稱軸是直線x= -1
C(c, 0)
-1 = (c + 4)/2
c = -6
y = a(x + 6)(x - 4)
x = 0, y = -24a = -4k, k = 6a
AB的斜率為k, BD的斜率為-1/k = (-4k - 0)/(0 + 1) = -4k
k? = 1/4
k = -1/2 (舍去k = 1/2 > 0)
a = k/6 = -1/12
y = -(x + 6)(x - 4)/12 = -x?/12 - x/6 + 2