Recall that a (differentiable) function f ( x ) {\displaystyle f(x)} is increasing on an interval if f ′ ( x ) ≥ 0 {\displaystyle f'(x)\geq 0} on that interval. Similarly, f ( x ) {\displaystyle f(x)} is decreasing on an interval if f ′ ( x ) ≤ 0 {\displaystyle f'(x)\leq 0} on that interval.
