<s>[INST] How to prove using induction that $ \sum_{i = 1}^n \frac{1}{\sum_{n = 0}^i n} = \frac{2n}{n+1}$ Using Mathematical Induction I need to prove that
$ \sum_{i = 1}^n \frac{1}{\sum_{n = 0}^i n}  =  \frac{2n}{n+1}$
As A first step I verified it for numbers 1 and 2 which worked
Secondly I simplified the LHS as:
$ \frac{1}{\sum n}  = \frac{1}{\frac {n(n+1)}{2}}  =  \frac{2}{n(n+1)}$
which converts my problem into
$ \frac{2}{1 \cdot 2} + \frac{2}{2 \cdot 3} + \frac{2}{3 \cdot 4} ......... + \frac{2}{n \cdot (n+1)} = \frac{2n}{n+1} $
After which I cancel out 2 as the common factor from both sides. But at this stage I am stuck and am unable to go forward. What should I do next ?
 [/INST] To do this by induction,
I start at the OP's simplification
$\sum_{k=1}^n \dfrac1{k(k+1)}
=\dfrac{n}{n+1}
$.
This is true for
$n=1$,
where it says
$\frac12 = \frac12$.
If it is true for $n$,
then
$\begin{array}\\
\sum_{k=1}^{n+1} \dfrac1{k(k+1)}
&=\sum_{k=1}^{n} \dfrac1{k(k+1)}+\dfrac1{(n+1)(n+2)}\\
&=\dfrac{n}{n+1}+\dfrac1{(n+1)(n+2)}\\
&=\dfrac{n(n+2)+1}{(n+1)(n+2)}\\
&=\dfrac{n^2+2n+1}{(n+1)(n+2)}\\
&=\dfrac{(n+1)^2}{(n+1)(n+2)}\\
&=\dfrac{n+1}{n+2}\\
\end{array}
$
and it is true for $n+1$.
Therefore it is true for all $n$.
</s>