C1

f (x) = 2-x-x^3
Show that f (x) is decreasing for all values of x. (4 marks)
The mark scheme states:
dy/dx = -3x^2 -1
x^2 is greater than 0 for all real values of x.
*-1-3x^2 is less than or equal to -1.
Hence f (x) is decreasing for all values of x.
I don't understand why dy/dx is less or equal to -1.