It is proved that the function f (x) = - x ^ 3 + 1 is a monotone decreasing function on R

It is proved that the function f (x) = - x ^ 3 + 1 is a monotone decreasing function on R

prove:
Set X10
(x2^2+x1x2+x1^2)>0
So f (x1) > F (x2)
So it's a monotone decreasing function