# monotony of a function

#### awsomeman

##### New member
Let $f$ be differentiable from $(-\inf,0)$ to $(0,\inf)$ and let $f'(x)<0$ for all real numbers except 0 and $f'(0)=0$. Prove that f is strictly decreasing.

#### Joppy

##### Well-known member
MHB Math Helper
You might want to begin by stating the definition of a decreasing function. Then consider some examples.