f(x)=-2x+3

Let x1> x2

f(x 1)-f(x2)

=-2x 1+3-(-2x2+3)

=-2x 1+2x2

= 2(x2-x 1)& lt； 0

Therefore: f (x 1)

So: f(x)=-2x+3 is a decreasing function on R.

10. Solution:

F(x) is the decreasing function of r, f(x? +2)& lt； Female (3x)

So: x? +2 & gt； 3x

So: x? -3x+2 >0

So: (x- 1)(x-2)>0.

So: x < 1 or x>2.