Need to find a convergent value

[Werg22]

Given this sum

s = \sum_{k = 1}^{{\frac{x}{j}} - 1} k^{n}j^{n+1}

x and n are constants

and x/j is a positive integrer and k is an integrer

To what value s converges as

{j}{\rightarrow}{0}
?

Edit: I have found that the awnser is \frac{x^{n+1}}{n+1}, but i do not know how to obtain this...

 

[Timbuqtu]

What do you recognize if I write
s = \sum_{k=1}^{N-1} x_k^n \Delta x
where N = x/(\Delta x) and x_k = k \Delta x? What about a Riemann sum?