$\dfrac{n+2n+...+nn}{n^3+n} \leq \dfrac{n}{n^3+1} + \dfrac{2n}{n^3+2}+...+ \dfrac{nn}{n^3+n} \leq \dfrac{n+2n+...+nn}{n^3+1}$
$\Rightarrow \dfrac{n.n.(n+1)}{2.(n^3+n)} \leq \dfrac{n}{n^3+1} + \dfrac{2n}{n^3+2}+...+ \dfrac{nn}{n^3+n} \leq \dfrac{n.n.(n+1)}{2n^3} $
$\Rightarrow \dfrac{n.(n+1)}{2.(n^2+1)} \leq \dfrac{n}{n^3+1} + \dfrac{2n}{n^3+2}+...+ \dfrac{nn}{n^3+n} \leq \dfrac{n.(n+1)}{2n^2} $
Mà
lim $\dfrac{n.(n+1)}{2.(n^2+1)} = lim \dfrac{n.(n+1)}{2n^2} = \dfrac{1}{2} $
$\Rightarrow Lim[\dfrac{n}{n^3+1} + \dfrac{2n}{n^3+2}+...+ \dfrac{nn}{n^3+n}] = \dfrac{1}{2} $