二次函数f(x)的图象与x轴交于(-1,0),(3,0),于是可以表达为f(x) = a(x + 1)(x - 3)
代入C(1,-8):a*2*(-2) = -8
a = 2
f(x) = 2(x + 1)(x - 3),图象开口向上,f(x) ≥ 0的解集为x ≥ 3或x ≤ -1