Prove that the sequence (why do they call it a sequence?):

$\displaystyle \sum_{k=n}^{3n}\frac{1}{k}$ has a limit 'a', which is in the range [$\displaystyle \frac{2}{3},\frac{3}{2}$]

I don't even know how to look at it : as a series, or as a sequence. I know it should be really easy, and should use some plain lemmas or convergence tests, I just don't know which one.

Thanks!