If $\lim_{x\rightarrow\infty}(f(x+1)-f(x))=L$ prove that $f=O(x)$. Let $f:\mathbb{R}\rightarrow\mathbb{R}$ such that 
$$\lim_{x\rightarrow\infty}(f(x+1)-f(x))=L$$
Prove that $$\lim_{x\rightarrow \infty}\dfrac{f(x)}{x}=L$$
This was an exam question that I was given and got nowhere on it. Going back now, I don't think I'm any closer. 
This is my idea so far. We know that 
$$\lim_{x\rightarrow\infty}\dfrac{f(x+1)-f(x)}{x}=0$$  
I think I'm supposed to add the apprapraite $0$ to
$$\left\vert \dfrac{f(x+1)-f(x)}{x}\right\vert$$ but I just keep getting a lower bound. 
A hint would be much appreciated. Thanks 

You need to impose that $f$ maps bounded intervals onto bounded intervals:
By the way, the proof by contraposition seems to be the more appropriate. But first, we recall the basic identity
$$ (f(x+1)-f(x)-L)^2=\left([f(x+1)-L(x+1)]+[Lx-f(x)]\right)^2= \\ \ \\
=(f(x+1)-L(x+1))^2+(f(x)-Lx)^2- \\ \ \\ -2(f(x)-Lx)(f(x+1)-L(x+1)).
$$
So, if $\displaystyle \lim_{x \rightarrow \infty} \frac{f(x)}{x} \neq L$, one can find $\varepsilon_0,\varepsilon_1>0$ s.t. for every $\delta>0$, one can find $x>\delta$ satisfying the set of inequalities
$$ |f(x)-Lx|\geq \varepsilon_0 |x| ~~~\&~~~ |f(x+1)-L(x+1)|\geq \varepsilon_1 |x+1| .$$
Now pick $\varepsilon=\min \{ \varepsilon_0,\varepsilon_1\}$. Then one gets 
$$(f(x)-Lx)^2\geq \varepsilon^2 x^2~~~\& ~~~~ (f(x)-Lx)^2\geq \varepsilon^2 (x+1)^2 $$
so that
$$
(f(x+1)-f(x)-L)^2\geq \varepsilon^2(x+1)^2+\varepsilon^2 x^2-2|x||x+1|\varepsilon^2.
$$

Remark: Although it is not said, the inequality $-2(f(x)-Lx)(f(x+1)-L(x+1))\geq -2\varepsilon^2 |x|~|x+1|$ only
  fulfils only in case that $f$ maps a bounded interval onto a bounded
  interval. That is the case when one impose the condition $x>\delta$
  [the set $f(]\delta,\infty[)$ is always bounded]. That means that the set of inequalities $f(x)-Lx\leq -\varepsilon |x|$ and $f(x+1)-L(x+1)\leq -\varepsilon |x+1|$ are always satisfied.

Finally, a short computation based on the binomial identity $(a-b)^2=a^2+b^2-2ab$ [$a=\varepsilon |x|$ & $b=\varepsilon |x+1|$] allows us to conclude that
$$
(f(x+1)-f(x)-L)^2\geq  \varepsilon^2.
$$
From the arbitrary of $\delta>0$, that is equivalent to say that
$\displaystyle \lim_{x \rightarrow \infty} (f(x+1)-f(x)) \neq L$.
In conclusion, we have proved that $\displaystyle \lim_{x \rightarrow \infty} \frac{f(x)}{x} \neq L$ implies that $\displaystyle \lim_{x \rightarrow \infty} (f(x+1)-f(x)) \neq L$. 
