How to prove that $\frac{m^{n+1}}{n+1}<1^n+2^n+\dots+m^n<(1+\frac{1}{m})^{n+1}\frac{m^{n+1}}{n+1} $? The first part of the problem is:
Prove that for all integers $n \ge 1$ and real numbers $t>1$,
$$ (n+1)t^n(t-1)>t^{n+1}-1>(n+1)(t-1)$$
I have done the first part by induction on $n$ for any real $t>1$.
However, I don't know how to do the second part, which is:
Use this to prove that if $m$ and $n$ are positive integers,
$$ \frac{m^{n+1}}{n+1}<1^n+2^n+\dots+m^n<\Big(1+\frac{1}{m}\Big)^{n+1}\frac{m^{n+1}}{n+1} $$
I factorized $t^{n+1}-1$ into $(t-1)(1+t+t^2+\dots+t^n)$ and cancelled $t-1$ to obtain $(n+1)t^n>1+t+t^2+\dots+t^n>n+1$. And have no idea what to do next. This is the only way I could think of in order to get a "sum", but couldn't see any relation between the two sum (if there are any...).
The last part of the question is:
Find
$$ \lim_{m\rightarrow \infty} \frac{1^n+2^n+\dots+m^n}{m^{n+1}}$$
I think I know how to do the last part. It should be divide the inequality by $m^{n+1}$, then apply Squeeze Theorem. I believe the answer is $\frac{1}{n+1}$.

Let $S=1^n+2^n+3^n+\cdots+m^n$. 
From a quick graph sketch it is clear that:
$$\begin{align}
\int_0^m x^n dx \;\;&<\qquad \qquad \quad S &&< \int_0^m (x+1)^n dx\\
\left[\frac {x^{n+1}}{n+1}\right]_0^m\;\;&<\qquad \qquad \quad S &&<\;\;\left[\frac {(x+1)^{n+1}}{n+1}\right]_0^m\\
\frac {m^{n+1}}{n+1}\;\;&<\qquad \qquad \quad S &&<\;\;\frac {(m+1)^{n+1}}{n+1}-\frac 1{n+1}\\
\frac {m^{n+1}}{n+1}\;\;&<\qquad \qquad \quad S &&<\;\;\left[\left(1+\frac 1m\right)^{m+1}-1\right]\frac {m^{n+1}}{n+1}<\left(1+\frac 1m\right)^{m+1}\frac {m^{n+1}}{n+1}\\
\frac {m^{n+1}}{n+1}\;\;&<\qquad1^n+2^n+3^n+\cdots+m^n &&<\left(1+\frac 1m\right)^{m+1}\frac {m^{n+1}}{n+1}\\
\end{align}$$
