Encontrar $\frac{1}{S_1}+\frac{1}{S_2}+\frac{1}{S_3}+\cdots+\frac{1}{S_{2013}}$

Question

Supongamos que $S_1=1 ,S_2=1+2,S=1+2+3+,\ldots,S_n=1+2+3+\cdots+n$

Cómo encontrar : $$\frac{1}{S_1}+\frac{1}{S_2}+\frac{1}{S_3}+\cdots+\frac{1}{S_{2013}}$$

Answer 1

Pista 1:

$$ \sum_{k=1}^n k = \frac{n (n+1)}{2} $$

Pista 2:

$$ \frac{1}{n (n+1)} = \frac{1}{n} - \frac{1}{n+1} $$

Answer 2

Puedes ver $ s_{n}=1+2+\cdots=\dfrac{n(n+1)}{2}$ , así que

$\dfrac{1}{s_{n}}=\dfrac{2}{n(n+1)}=2\left(\dfrac{1}{n}-\dfrac{1}{n+1}\right)$

entonces

$\dfrac{1}{s_{1}}+\dfrac{1}{s_{2}}+\cdots+\dfrac{1}{s_{2013}}=2\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\cdots+\dfrac{1}{2013}-\dfrac{1}{2014}\right)=\dfrac{2013}{1007}$

Answer 3

$$\sum_{n=1}^{2013}\frac{1}{n(n+1)/2}=\sum_{n=1}^{2013}\frac{2}{n(n+1)}=2\sum_{n=1}^{2013}\frac{1}{n(n+1)}=2\sum_{n=1}^{2013}\left(\frac{1}{n}-\frac{1}{n+1}\right)=$$ $$2\sum_{n=1}^{2013}\frac{1}{n}-2\sum_{n=1}^{2013}\frac{1}{n+1}=2\sum_{n=1}^{2013}\frac{1}{n}-2\sum_{n=2}^{2014}\frac{1}{n}=2\sum_{n=1}^{2013}\frac{1}{n}-2\left(\sum_{n=1}^{2013}\frac{1}{n}-1+\frac{1}{2014}\right)=$$ $$=2-\frac{2}{2014}$$