Let be a differentiable function and is derivative function. Properties if f' give us information about f:

1. If f' (x)>0 for way ,then f (x) strictly increasing in (c, d).
2. If f' (x) < 0 for way ,then f (x) is strictly decreasing in (c, d).
3. If f' (x)=0 then f has a horizontal tangent at x and possible a point of local extreme.