I'll give you a hint. If you still can't do it, I can post a full solution:
1. Assume it true for n=k
2. Test n=k+1
3. Sub in your assumption to get:
<a href="http://www.codecogs.com/eqnedit.php?latex=1@plus;\frac{1}{\sqrt2}@plus;...@plus;\frac{1}{\sqrt{k}}@plus;\frac{1}{\sqrt{k@plus;1}}>\frac{1}{\sqrt{k@plus;1}}@plus;2(\sqrt{k@plus;1}-1)" target="_blank"><img src="http://latex.codecogs.com/gif.latex?1+\frac{1}{\sqrt2}+...+\frac{1}{\sqrt{k}}+\frac{1}{\sqrt{k+1}}>\frac{1}{\sqrt{k+1}}+2(\sqrt{k+1}-1)" title="1+\frac{1}{\sqrt2}+...+\frac{1}{\sqrt{k}}+\frac{1}{\sqrt{k+1}}>\frac{1}{\sqrt{k+1}}+2(\sqrt{k+1}-1)" /></a>
4. Now put the RHS on the same denominator and try and go from there...