<s>[INST] Sum of an infinite series. I have found a series whose $n$-th term is denoted by $\dfrac{n}{a^n}$. Here $a$ is a constant. I tried to find the sum but failed. Is there any formula for its infinite sum or just an approximation?
 [/INST] notice, we have $$T_n=\frac{n}{a^n}$$ $$S_n=\sum_{n=1}^{n} T_n=\sum_{n=1}^{n}\frac{n}{a^n}$$ $$S_n=\frac{1}{a}+\frac{2}{a^2}+\frac{3}{a^3}+\frac{4}{a^4}+\ldots +\frac{n}{a^n}\tag 1$$ Multiplying by $\frac{1}{a}$ &  rewriting as follows
$$\frac{1}{a}S_n= \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{1}{a^2}+\frac{2}{a^3}+\frac{3}{a^4}+\frac{4}{a^5}+\ldots +\frac{n-1}{a^{n}}+\frac{n}{a^{n+1}}\tag 2$$ Now, subtracting the corresponding terms of (2) from (1) column wise, we get 
$$S_n-\frac{1}{a}S_n=\underbrace{\frac{1}{a}+\frac{1}{a^2}+\frac{1}{a^3}+\ldots+\frac{1}{a^n}}_{\text{n-terms in G.P.}}-\frac{n}{a^{n+1}}$$
$$\left(\frac{a-1}{a}\right)S_n=\frac{\frac{1}{a}\left(\frac{1}{a^n}-1\right)}{\frac{1}{a}-1}-\frac{n}{a^{n+1}}$$
$$S_n=\left(\frac{a}{a-1}\right)\left\{\frac{a^n-1}{a^n(a-1)}-\frac{n}{a^{n+1}}\right\}$$
Hence, taking the limit as $n\to \infty$, the sum of infinite terms $S_{\infty}$ is given as follows $$S_{\infty}=\lim_{n\to \infty}S_n$$
$$=\lim_{n\to \infty}\left(\frac{a}{a-1}\right)\left\{\frac{a^n-1}{a^n(a-1)}-\frac{n}{a^{n+1}}\right\}$$
$$=\left(\frac{a}{a-1}\right)\lim_{n\to \infty}\left\{\frac{a^n-1}{a^n(a-1)}-\frac{n}{a^{n+1}}\right\}$$
$$S_{\infty}=\frac{1}{(a-1)^2}\lim_{n\to \infty}\left\{\frac{a^{n+1}-n(a-1)-a}{a^n}\right\}$$
It is obvious that the series converses for $|a|\leq 1$ otherwise diverges
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