That is, ax 2+bx-4 = a * (x) 2+b * (x)-4 = ax 2-bx-4.

So b = 0, f (x) = ax 2-4,

And because (1, -3), F (1) = A * 1 2-4 =-3, so A = 1, f (x) = x 2-4.

So f(x) decreases at (-∞, 0) and increases at 0,+∞).

19, (1) From the meaning of the question, the distance of 20km means more than 4km, and the excess part is16km.

Therefore, the excess should be (20-4)* 1.2= 19.2 yuan, and the first 4 kilometers are 10 yuan, * * Payment19.2+10 =19.2 yuan (.

y = 10(x & lt； =4)

y = 1.2 *(x-4)( 1 & lt； x & lt=22)

20. (1) According to the meaning of the question, f (1) = 1+m = 5, m = 4, and f (x) = x+4/x (x ≠ 0).

(2) because f(x)=x+4/x, f(-x)=-x+4/(-x)=-(x+4/x)=-f(x)

So f(x) is the odd function in the definition field.

(3) x 1, x2 in (2, +∞) and x 1

So f (x1)-f (x2) = x1+4/x1-(x2+4/x2) = (x1-x2) [1-4/(x

Because x 1, x2 >；; 2, so x1x 2 >; 4，4/(x 1x 2)& lt； 1， 1-4/(x 1x2) >: 0

So f (x 1)-f (x2) < 0, that is, f (x 1) < f(x2),

So the function is increasing function at (2, +∞).