Sum of series n(x^n)

The solution

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  \      n
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n = 1

$$\sum_{n=1}^{\infty} n x^{n}$$

Sum(n*x^n, (n, 1, oo))

The radius of convergence of the power series

Given number:
$$n x^{n}$$
It is a series of species
$$a_{n} \left(c x - x_{0}\right)^{d n}$$
- power series.
The radius of convergence of a power series can be calculated by the formula:
$$R^{d} = \frac{x_{0} + \lim_{n \to \infty} \left|{\frac{a_{n}}{a_{n + 1}}}\right|}{c}$$
In this case
$$a_{n} = n$$
and
$$x_{0} = 0$$
,
$$d = 1$$
,
$$c = 1$$
then
$$R = \lim_{n \to \infty}\left(\frac{n}{n + 1}\right)$$
Let's take the limit
we find
$$R = 1$$

The answer [src]

/    x                  
| --------   for |x| < 1
|        2              
| (1 - x)               
|                       
|  oo                   
< ___                   
| \  `                  
|  \      n             
|  /   n*x    otherwise 
| /__,                  
|n = 1                  
\

$$\begin{cases} \frac{x}{\left(1 - x\right)^{2}} & \text{for}\: \left|{x}\right| < 1 \\\sum_{n=1}^{\infty} n x^{n} & \text{otherwise} \end{cases}$$

Piecewise((x/(1 - x)^2, |x| < 1), (Sum(n*x^n, (n, 1, oo)), True))

Examples of finding the sum of a series

The Sum of the Power Series
```
x^n/n
```
```
(x-1)^n
```
Factorial
```
1/2^(n!)
```
```
n^2/n!
```
```
x^n/n!
```
```
k!/(n!*(n+k)!)
```
Flint Hills Series
```
csc(n)^2/n^3
```
Basel problem
```
1/n^2
```
```
1/n^4
```
```
1/n^6
```
Harmonic series
```
1/n
```
Grandi's series
```
(-1)^n
```
Alternating series
```
(-1)^(n + 1)/n
```
```
(n + 2)*(-1)^(n - 1)
```
```
(3*n - 1)/(-5)^n
```
```
(-1)^(n - 1)*n/(6*n - 5)
```
Newton–Mercator series
```
(-1)^(n + 1)/n*x^n
```
Exam the series for convergence
```
(3*n - 1)/(-5)^n
```

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Identical expressions

Sum of series n(x^n)

The solution

Examples of finding the sum of a series