4)
该函数与 x 轴交点 A(x1,0),B (x2,0)
|AB|= x2 - x1 = 6
x2 + x1 = -b/a = 2
x2 = (6+2)/2 = 4,x1 = (2-6)/2 = -2
A (-2,0),B (4,0)
5)
当 x > 2 时,|x+1| + |x-2| = x + 1 + x - 2 = 2x -1 > 3
当 x < -1 时,|x+1| + |x-2| = -x - 1 - (x - 2) = -2x + 1 >3
当 -1