Prove that $\lim(x_n)=0$ if and only if $\lim(|x_n|)=0$. 
Prove that $\lim(x_n)=0$ if and only if $\lim(|x_n|)=0$.

Definition:
Let $X = (x_n)$ be a sequence in $\mathbb{R}$ and let $x\in\mathbb{R}$.
Then $\lim(x_n) =x$ iff for all $\varepsilon>0$, $\exists k\in\mathbb{N}$ such that $|x_n-x|<\varepsilon$ for all $n\geq k$.
I am wondering if this is sufficient:
If we know that $\lim(x_n)=0$, we know for all $\varepsilon>0$, $\exists k\in\mathbb{N}$ such that $|x_n-0|<\varepsilon$ for all $n\geq k$.
We can rewrite this as the following:
$$|x_n-0|<\varepsilon \Leftrightarrow ||x_n|-0|<\varepsilon\Leftrightarrow \lim(|x_n|)=0$$
Then the conclusion seems to follow logically. Is the proof really that simple or are there some things I am missing?

Your proof is correct. Note that we also have (in general)
$$\lim x_n = c \Rightarrow \lim |x_n| = |c|$$ 
and that $\lim |x_n| = c$ does not in general imply that $\lim x_n$ exists. 
