How find this $\lim_{n\to\infty}n^2\left(\frac{1^k+2^k+\cdots+n^k}{n^{k+1}}-\frac{1}{k+1}-\frac{1}{2n}\right)$ Find this limit

$$\lim_{n\to\infty}n^2\left(\dfrac{1^k+2^k+\cdots+n^k}{n^{k+1}}-\dfrac{1}{k+1}-\dfrac{1}{2n}\right)\tag{1}$$

I can  only solve this limit

$$I=\lim_{n\to\infty}\left(\dfrac{1^k+2^k+\cdots+n^k}{n^k}-\dfrac{n}{k+1}\right)=\dfrac{1}{2}$$
  proof:
  use Stolz therom:let
  $$a_{n}=(k+1)(1^k+2^k+\cdots+n^k)-n^{k+1},b_{n}=(k+1)n^k$$
  then
  $$I=\lim_{n\to\infty}\dfrac{a_{n}}{b_{n}}=\lim_{n\to\infty}\dfrac{a_{n+1}-a_{n}}{b_{n+1}-b_{n}}=\dfrac{1}{k+1}\dfrac{(k+1)(n+1)^k-(n+1)^{k+1}+n^{k+1}}{(n+1)^k-n^k}$$
  so
  $$I=\lim_{n\to\infty}\dfrac{1}{k+1}\dfrac{[k(k+1)/2]n^{k-1}+\cdots+k}{kn^{k-1}+\cdots+1}=\dfrac{1}{2}$$
  But for (1),I can't,Thanks.


For every $k\geqslant0$ and $n\geqslant0$, let 
$$
s_n^k=\sum\limits_{i=1}^ni^k.
$$
We first recall how to get a simple equivalent of $s_n^k$ when $n\to\infty$. One starts from the fact that, for every $i\geqslant1$, $(i-1)^k\leqslant t^k\leqslant i^k$ for every $t$ in $(i-1,i)$. Summing this from $i=1$ to $i=n$ yields
$$
(k+1)s_{n-1}^k\leqslant (k+1)\int_0^nt^k\mathrm dt\leqslant(k+1)s_n^k,
$$
that is,
$$
n^{k+1}\leqslant (k+1)s_n^k=(k+1)s_{n-1}^k+(k+1)n^k\leqslant n^{k+1}+(k+1)n^k,
$$
which is enough to get
$$
s_n^k=\frac{n^{k+1}}{k+1}+O(n^k)\tag{$\dagger$}.
$$
To be more precise than $(\dagger)$, one needs to refine the inequalities $(i-1)^k\leqslant t^k\leqslant i^k$. For every $i\geqslant1$ and every $t$ in $(i-1,i)$, Taylor formula reads
$$
t^k=i^k+ki^{k-1}(t-i)+\frac12k(k-1)r^{k-2}(t-i)^2,
$$
for some $r$ in $(t,i)$. If $k\geqslant2$, $r^{k-2}\leqslant i^{k-2}$. Furthermore, for every $i\geqslant1$, $\int\limits_{i-1}^i(t-i)\mathrm dt=-\frac12$ and $\int\limits_{i-1}^i(t-i)^2\mathrm dt=\frac13$ hence, integrating both sides from $t=i-1$ to $t=i$ and summing over $i$ from $i=1$ to $i=n$ yields
$$
n^{k+1}=(k+1)s_n^k-\frac12(k+1)ks_n^{k-1}+\frac16(k+1)k(k-1)s_n^{k-2}\theta_n^k,
$$
where $\theta_n^k$ is in $(0,1)$. Since $ks_n^{k-1}=n^k+O(n^{k-1})$ and $s_n^{k-2}=O(n^{k-1})$, this guarantees that
$$
s_n^k=\frac{n^{k+1}}{k+1}+\frac12n^k+O(n^{k-1}).\tag{$\ddagger$}
$$
The expansion $(\ddagger)$ refines $(\dagger)$ but we need one more term. To get it, use $(\ddagger)$ for $k-1$. This reads
$$
ks_n^{k-1}=n^{k}+\frac12kn^{k-1}+O(n^{k-2}),
$$ 
Additionally, note that $s_n^{k-1}\leqslant s_n^{k-2}\theta_n^k\leqslant s_n^{k-2}$ hence $\theta_n^k\to1$ when $n\to\infty$.; Since $(k-1)s_n^{k-2}n^{k-1}+O(n^{k-2})$, putting everything together yields
$$
n^{k+1}=(k+1)s_n^k-\frac12(k+1)\left(n^{k}+\frac12kn^{k-1}\right)+\frac16(k+1)kn^{k-1}+O(n^{k-2}),
$$
that is,
$$
\frac{n^{k+1}}{k+1}=s_n^k-\frac12n^{k}-\frac1{12}kn^{k-1}+O(n^{k-2}),
$$
which is equivalent to
$$
n^2\left(\frac{s_n^k}{n^{k+1}}-\frac1{k+1}-\frac1{2n}\right)=\frac1{12}k+O\left(\frac1n\right).
$$
