It's sufficient enough to show, that $|\sqrt{n+1}|-|\sqrt{n}|$ convergences, since all convergent sequences are Cauchy sequences. What I've done so far:
$$||\sqrt{n+1}|-|\sqrt{n}||= |\sqrt{n+1}|-|\sqrt{n}|=\sqrt{n+1}-\sqrt{n}$$
since $n\in\mathbb{N}$ , so $\sqrt{n+1}$ and $\sqrt{n}$ are positive.
After that I can't find an upper estimate so I eventually arrive at $<\epsilon$.
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