Let $f$ be a non-negative differentiable function such that $f'$ is continuous and $\int_{0}^{\infty}f(x)\,dx$ and $\int_{0}^{\infty}f'(x)\,dx$ exist. 
Let $f$ be a non-negative differentiable function such that $f'$ is continuous and
  $\displaystyle\int_{0}^{\infty}f(x)\,dx$ and $\displaystyle\int_{0}^{\infty}f'(x)\,dx$ exist.
Prove or give a counter example: $f'(x)\overset{x\rightarrow
\infty}{\rightarrow} 0$

Note: I think it is not true but I couldn't find a counter example.

Let $g_n(x) = \sin (n^2 x) 1_{[0,{2 \pi \over n^2}]}$ and note that $g_n$ is continuous, $\int |g_n| = {2 \over n^2}$, $\int g_n = 0$ and if 
$f_n(x)=\int_0^x g_n$, then $f_n(x) \ge 0$ for all $x$. Furthermore,
$\int |f_n| \le {4 \pi \over n^3}$.
Define $f(x) = \sum_{n \ge 1} f_n(x-n)$, note that $f$ is integrable and non negative. Furthermore, $f'(x) = \sum_{n \ge 1} g_n(x-n)$ and it is straightforward to check that $f'$ is integrable as well.
